middle-end: adjust loop upper bounds when peeling for gaps and early break [PR114403].

Hi All,

This is a story all about how the peeling for gaps introduces a bug in the upper
bounds.

Before I go further, I'll first explain how I understand this to work for loops
with a single exit.

When peeling for gaps we peel N < VF iterations to scalar.
This happens by removing N iterations from the calculation of niters such that
vect_iters * VF == niters is always false.

In other words, when we exit the vector loop we always fall to the scalar loop.
The loop bounds adjustment guarantees this. Because of this we potentially
execute a vector loop iteration less.  That is, if you're at the boundary
condition where niters % VF by peeling one or more scalar iterations the vector
loop executes one less.

This is accounted for by the adjustments in vect_transform_loops.  This
adjustment happens differently based on whether the the vector loop can be
partial or not:

Peeling for gaps sets the bias to 0 and then:

when not partial:  we take the floor of (scalar_upper_bound / VF) - 1 to get the
		   vector latch iteration count.

when loop is partial:  For a single exit this means the loop is masked, we take
                       the ceil to account for the fact that the loop can handle
		       the final partial iteration using masking.

Note that there's no difference between ceil an floor on the boundary condition.
There is a difference however when you're slightly above it. i.e. if scalar
iterates 14 times and VF = 4 and we peel 1 iteration for gaps.

The partial loop does ((13 + 0) / 4) - 1 == 2 vector iterations. and in effect
the partial iteration is ignored and it's done as scalar.

This is fine because the niters modification has capped the vector iteration at
2.  So that when we reduce the induction values you end up entering the scalar
code with ind_var.2 = ind_var.1 + 2 * VF.

Now lets look at early breaks.  To make it esier I'll focus on the specific
testcase:

char buffer[64];

__attribute__ ((noipa))
buff_t *copy (buff_t *first, buff_t *last)
{
  char *buffer_ptr = buffer;
  char *const buffer_end = &buffer[SZ-1];
  int store_size = sizeof(first->Val);
  while (first != last && (buffer_ptr + store_size) <= buffer_end)
    {
      const char *value_data = (const char *)(&first->Val);
      __builtin_memcpy(buffer_ptr, value_data, store_size);
      buffer_ptr += store_size;
      ++first;
    }

  if (first == last)
    return 0;

  return first;
}

Here the first, early exit is on the condition:

  (buffer_ptr + store_size) <= buffer_end

and the main exit is on condition:

  first != last

This is important, as this bug only manifests itself when the first exit has a
known constant iteration count that's lower than the latch exit count.

because buffer holds 64 bytes, and VF = 4, unroll = 2, we end up processing 16
bytes per iteration.  So the exit has a known bounds of 8 + 1.

The vectorizer correctly analizes this:

Statement (exit)if (ivtmp_21 != 0)
 is executed at most 8 (bounded by 8) + 1 times in loop 1.

and as a consequence the IV is bound by 9:

  # vect_vec_iv_.14_117 = PHI <_118(9), { 9, 8, 7, 6 }(20)>
  ...
  vect_ivtmp_21.16_124 = vect_vec_iv_.14_117 + { 18446744073709551615, 18446744073709551615, 18446744073709551615, 18446744073709551615 };
  mask_patt_22.17_126 = vect_ivtmp_21.16_124 != { 0, 0, 0, 0 };
  if (mask_patt_22.17_126 == { -1, -1, -1, -1 })
    goto <bb 3>; [88.89%]
  else
    goto <bb 30>; [11.11%]

The imporant bits are this:

In this example the value of last - first = 416.

the calculated vector iteration count, is:

    x = (((ptr2 - ptr1) - 16) / 16) + 1 = 27

the bounds generated, adjusting for gaps:

   x == (((x - 1) >> 2) << 2)

which means we'll always fall through to the scalar code. as intended.

Here are two key things to note:

1. In this loop, the early exit will always be the one taken.  When it's taken
   we enter the scalar loop with the correct induction value to apply the gap
   peeling.

2. If the main exit is taken, the induction values assumes you've finished all
   vector iterations.  i.e. it assumes you have completed 24 iterations, as we
   treat the main exit the same for normal loop vect and early break when not
   PEELED.
   This means the induction value is adjusted to ind_var.2 = ind_var.1 + 24 * VF;

So what's going wrong.  The vectorizer's codegen is correct and efficient,
however when we adjust the upper bounds, that code knows that the loops upper
bound is based on the early exit. i.e. 8 latch iterations. or in other words.
It thinks the loop iterates once.

This is incorrect as the vector loop iterates twice, as it has set up the
induction value such that it exits at the early exit.   So it in effect iterates
2.5x times.

Becuase the upper bound is incorrect, when we unroll it now exits from the main
exit which uses the incorrect induction value.

So there are three ways to fix this:

1.  If we take the position that the main exit should support both premature
    exits and final exits then vect_update_ivs_after_vectorizer needs to be
    skipped for this case, and vectorizable_induction updated with  third case
    where we reduce with LAST reduction based on the IVs instead of assuming
    you're at the end of the vector loop.

    I don't like this approach.  It don't think we should add a third induction
    style to cover up an issue introduced by unrolling.  It makes the code
    harder to follow and makes main exits harder to reason about.

2. We could say that vec_init_loop_exit_info should pick the exit which has the
   smallest known iteration count.  This would turn this case into a PEELED case
   and the induction values would be correct as we'd always recalculate them
   from a reduction.  This is suboptimal though as the reason we pick the latch
   exit as the IV one is to prevent having to rotate the loop.  This results
   in more efficient code for what we assume is the common case, i.e. the main
   exit.

3. In PR113734 we've established that for vectorization of early breaks that we
   must always treat the loop as partial.  Here partiallity means that we have
   enough vector elements to start the iteration, but we may take an early exit
   and so never reach the latch/main exit.

   This requirement is overwritten by the peeling for gaps adjustment of the
   upper bound.  I believe the bug is simply that this shouldn't be done.
   The adjustment here is to indicate that the main exit always leads to the
   scalar loop when peeling for gaps.

   But this invariant is already always true for all early exits.  Remember that
   early exits restart the scalar loop at the start of the vector iteration, so
   the induction values will start it where we want to do the gaps peeling.

I think no# 3 is the correct fix, and also one that doesn't degrade code quality.

Note: I used memcpy and memcmp in the testcase, I'm not sure if I can rely on
      these being inlined? but I also don't know how to test for library support.

Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu and
no issues.

Ok for master?

Thanks,
Tamar

gcc/ChangeLog:

	PR tree-optimization/114403
	* tree-vect-loop.cc (vect_transform_loop): Adjust upper bounds for when
	peeling for gaps and early break.

gcc/testsuite/ChangeLog:

	PR tree-optimization/114403
	* gcc.dg/vect/vect-early-break_124-pr114403.c: New test.

---




--

On Fri, 12 Apr 2024, Tamar Christina wrote:

> Hi All,
> 
> This is a story all about how the peeling for gaps introduces a bug in the upper
> bounds.
> 
> Before I go further, I'll first explain how I understand this to work for loops
> with a single exit.
> 
> When peeling for gaps we peel N < VF iterations to scalar.
> This happens by removing N iterations from the calculation of niters such that
> vect_iters * VF == niters is always false.
> 
> In other words, when we exit the vector loop we always fall to the scalar loop.
> The loop bounds adjustment guarantees this. Because of this we potentially
> execute a vector loop iteration less.  That is, if you're at the boundary
> condition where niters % VF by peeling one or more scalar iterations the vector
> loop executes one less.
> 
> This is accounted for by the adjustments in vect_transform_loops.  This
> adjustment happens differently based on whether the the vector loop can be
> partial or not:
> 
> Peeling for gaps sets the bias to 0 and then:
> 
> when not partial:  we take the floor of (scalar_upper_bound / VF) - 1 to get the
> 		   vector latch iteration count.
> 
> when loop is partial:  For a single exit this means the loop is masked, we take
>                        the ceil to account for the fact that the loop can handle
> 		       the final partial iteration using masking.
> 
> Note that there's no difference between ceil an floor on the boundary condition.
> There is a difference however when you're slightly above it. i.e. if scalar
> iterates 14 times and VF = 4 and we peel 1 iteration for gaps.
> 
> The partial loop does ((13 + 0) / 4) - 1 == 2 vector iterations. and in effect
> the partial iteration is ignored and it's done as scalar.
> 
> This is fine because the niters modification has capped the vector iteration at
> 2.  So that when we reduce the induction values you end up entering the scalar
> code with ind_var.2 = ind_var.1 + 2 * VF.
> 
> Now lets look at early breaks.  To make it esier I'll focus on the specific
> testcase:
> 
> char buffer[64];
> 
> __attribute__ ((noipa))
> buff_t *copy (buff_t *first, buff_t *last)
> {
>   char *buffer_ptr = buffer;
>   char *const buffer_end = &buffer[SZ-1];
>   int store_size = sizeof(first->Val);
>   while (first != last && (buffer_ptr + store_size) <= buffer_end)
>     {
>       const char *value_data = (const char *)(&first->Val);
>       __builtin_memcpy(buffer_ptr, value_data, store_size);
>       buffer_ptr += store_size;
>       ++first;
>     }
> 
>   if (first == last)
>     return 0;
> 
>   return first;
> }
> 
> Here the first, early exit is on the condition:
> 
>   (buffer_ptr + store_size) <= buffer_end
> 
> and the main exit is on condition:
> 
>   first != last
> 
> This is important, as this bug only manifests itself when the first exit has a
> known constant iteration count that's lower than the latch exit count.
> 
> because buffer holds 64 bytes, and VF = 4, unroll = 2, we end up processing 16
> bytes per iteration.  So the exit has a known bounds of 8 + 1.
> 
> The vectorizer correctly analizes this:
> 
> Statement (exit)if (ivtmp_21 != 0)
>  is executed at most 8 (bounded by 8) + 1 times in loop 1.
> 
> and as a consequence the IV is bound by 9:
> 
>   # vect_vec_iv_.14_117 = PHI <_118(9), { 9, 8, 7, 6 }(20)>
>   ...
>   vect_ivtmp_21.16_124 = vect_vec_iv_.14_117 + { 18446744073709551615, 18446744073709551615, 18446744073709551615, 18446744073709551615 };
>   mask_patt_22.17_126 = vect_ivtmp_21.16_124 != { 0, 0, 0, 0 };
>   if (mask_patt_22.17_126 == { -1, -1, -1, -1 })
>     goto <bb 3>; [88.89%]
>   else
>     goto <bb 30>; [11.11%]
> 
> The imporant bits are this:
> 
> In this example the value of last - first = 416.
> 
> the calculated vector iteration count, is:
> 
>     x = (((ptr2 - ptr1) - 16) / 16) + 1 = 27
> 
> the bounds generated, adjusting for gaps:
> 
>    x == (((x - 1) >> 2) << 2)
> 
> which means we'll always fall through to the scalar code. as intended.
> 
> Here are two key things to note:
> 
> 1. In this loop, the early exit will always be the one taken.  When it's taken
>    we enter the scalar loop with the correct induction value to apply the gap
>    peeling.
> 
> 2. If the main exit is taken, the induction values assumes you've finished all
>    vector iterations.  i.e. it assumes you have completed 24 iterations, as we
>    treat the main exit the same for normal loop vect and early break when not
>    PEELED.
>    This means the induction value is adjusted to ind_var.2 = ind_var.1 + 24 * VF;
> 
> So what's going wrong.  The vectorizer's codegen is correct and efficient,
> however when we adjust the upper bounds, that code knows that the loops upper
> bound is based on the early exit. i.e. 8 latch iterations. or in other words.
> It thinks the loop iterates once.
> 
> This is incorrect as the vector loop iterates twice, as it has set up the
> induction value such that it exits at the early exit.   So it in effect iterates
> 2.5x times.
> 
> Becuase the upper bound is incorrect, when we unroll it now exits from the main
> exit which uses the incorrect induction value.
> 
> So there are three ways to fix this:
> 
> 1.  If we take the position that the main exit should support both premature
>     exits and final exits then vect_update_ivs_after_vectorizer needs to be
>     skipped for this case, and vectorizable_induction updated with  third case
>     where we reduce with LAST reduction based on the IVs instead of assuming
>     you're at the end of the vector loop.
> 
>     I don't like this approach.  It don't think we should add a third induction
>     style to cover up an issue introduced by unrolling.  It makes the code
>     harder to follow and makes main exits harder to reason about.
> 
> 2. We could say that vec_init_loop_exit_info should pick the exit which has the
>    smallest known iteration count.  This would turn this case into a PEELED case
>    and the induction values would be correct as we'd always recalculate them
>    from a reduction.  This is suboptimal though as the reason we pick the latch
>    exit as the IV one is to prevent having to rotate the loop.  This results
>    in more efficient code for what we assume is the common case, i.e. the main
>    exit.
> 
> 3. In PR113734 we've established that for vectorization of early breaks that we
>    must always treat the loop as partial.  Here partiallity means that we have
>    enough vector elements to start the iteration, but we may take an early exit
>    and so never reach the latch/main exit.
> 
>    This requirement is overwritten by the peeling for gaps adjustment of the
>    upper bound.  I believe the bug is simply that this shouldn't be done.
>    The adjustment here is to indicate that the main exit always leads to the
>    scalar loop when peeling for gaps.
> 
>    But this invariant is already always true for all early exits.  Remember that
>    early exits restart the scalar loop at the start of the vector iteration, so
>    the induction values will start it where we want to do the gaps peeling.
> 
> I think no# 3 is the correct fix, and also one that doesn't degrade code quality.
> 
> Note: I used memcpy and memcmp in the testcase, I'm not sure if I can rely on
>       these being inlined? but I also don't know how to test for library support.
> 
> Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu and
> no issues.
> 
> Ok for master?

I agree with the patch outcome.  I think the reasoning is simpler
because peeling for gaps applies to the scalar iteration IV but
with multiple exits an upper bound can effectively apply to a
different exit and thus a different "IV" we cannot simply adjust
an existing upper bound as if it were a bound for the scalar iteration IV.

So I think we should word it like the following.  Can you pick that
up and also add the other testcase I produced?

OK with those changes.

Thanks,
Richard.

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 3ffcac8c613..c728856079b 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -12152,14 +12152,14 @@ vect_transform_loop (loop_vec_info loop_vinfo, 
gimple 
*loop_vectorized_call)
   bool final_iter_may_be_partial
     = LOOP_VINFO_USING_PARTIAL_VECTORS_P (loop_vinfo)
       || LOOP_VINFO_EARLY_BREAKS (loop_vinfo);
-  /* The minimum number of iterations performed by the epilogue.  This
-     is 1 when peeling for gaps because we always need a final scalar
-     iteration.  */
-  int min_epilogue_iters = LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo) ? 1 : 
0;
-  /* +1 to convert latch counts to loop iteration counts,
-     -min_epilogue_iters to remove iterations that cannot be performed
-       by the vector code.  */
-  int bias_for_lowest = 1 - min_epilogue_iters;
+  /* +1 to convert latch counts to loop iteration counts.  */
+  int bias_for_lowest = 1;
+  /* When we are peeling for gaps then we take away one scalar iteration
+     from the vector loop.  Thus we can adjust the upper bound by one
+     scalar iteration.  But only when we know the bound applies to the
+     IV exit test which might not be true when we have multiple exits.  
*/
+  if (! LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
+    bias_for_lowest -= LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo) ? 1 : 0;
   int bias_for_assumed = bias_for_lowest;
   int alignment_npeels = LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo);
   if (alignment_npeels && LOOP_VINFO_USING_PARTIAL_VECTORS_P 
(loop_vinfo))


> Thanks,
> Tamar
> 
> gcc/ChangeLog:
> 
> 	PR tree-optimization/114403
> 	* tree-vect-loop.cc (vect_transform_loop): Adjust upper bounds for when
> 	peeling for gaps and early break.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR tree-optimization/114403
> 	* gcc.dg/vect/vect-early-break_124-pr114403.c: New test.
> 
> ---
> diff --git a/gcc/testsuite/gcc.dg/vect/vect-early-break_124-pr114403.c b/gcc/testsuite/gcc.dg/vect/vect-early-break_124-pr114403.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..ae5e53efc45e7bef89c5a72abd6afa48292668db
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/vect-early-break_124-pr114403.c
> @@ -0,0 +1,74 @@
> +/* { dg-add-options vect_early_break } */
> +/* { dg-require-effective-target vect_early_break_hw } */
> +/* { dg-require-effective-target vect_long_long } */
> +
> +/* { dg-final { scan-tree-dump "LOOP VECTORIZED" "vect" } } */
> +
> +#include "tree-vect.h"
> +
> +typedef unsigned long PV;
> +typedef struct _buff_t {
> +    int foo;
> +    PV Val;
> +} buff_t;
> +
> +#define NUM 9
> +#define SZ NUM * sizeof (PV)
> +char buffer[SZ];
> +
> +__attribute__ ((noipa))
> +buff_t *copy (buff_t *first, buff_t *last)
> +{
> +  char *buffer_ptr = buffer;
> +  char *const buffer_end = &buffer[SZ-1];
> +  int store_size = sizeof(first->Val);
> +  while (first != last && (buffer_ptr + store_size) <= buffer_end)
> +    {
> +      const char *value_data = (const char *)(&first->Val);
> +      __builtin_memcpy(buffer_ptr, value_data, store_size);
> +      buffer_ptr += store_size;
> +      ++first;
> +    }
> +
> +  if (first == last)
> +    return 0;
> +
> +  return first;
> +}
> +
> +int main ()
> +{
> +  /* Copy an ascii buffer.  We need to trigger the loop to exit from
> +     the condition where we have more data to copy but not enough space.
> +     For this test that means that OVL must be > SZ.  */
> +#define OVL NUM*2
> +  char str[OVL]="abcdefghiabcdefgh\0";
> +  buff_t tmp[OVL];
> +
> +#pragma GCC novector
> +  for (int i = 0; i < OVL; i++)
> +    tmp[i].Val = str[i];
> +
> +  buff_t *start = &tmp[0];
> +  buff_t *last = &tmp[OVL-1];
> +  buff_t *res = 0;
> +
> +  /* This copy should exit on the early exit, in which case we know
> +     that start != last as we had more data to copy but the buffer
> +     was full.  */
> +  if (!(res = copy (start, last)))
> +    __builtin_abort ();
> +
> +  /* Check if we have the right reduction value.  */
> +  if (res != &tmp[NUM-1])
> +    __builtin_abort ();
> +
> +  int store_size = sizeof(PV);
> +#pragma GCC novector
> +  for (int i = 0; i < NUM - 1; i+=store_size)
> +    if (0 != __builtin_memcmp (buffer+i, (char*)&tmp[i].Val, store_size))
> +      __builtin_abort ();
> +
> +  return 0;
> +}
> +
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index 4375ebdcb493a90fd0501cbb4b07466077b525c3..024a24a305c4727f97eb022247f4dca791c52dfe 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -12144,6 +12144,12 @@ vect_transform_loop (loop_vec_info loop_vinfo, gimple *loop_vectorized_call)
>       -min_epilogue_iters to remove iterations that cannot be performed
>         by the vector code.  */
>    int bias_for_lowest = 1 - min_epilogue_iters;
> +  /* For an early break we must always assume that the vector loop can be
> +     executed partially.  In this definition a partial iteration means that we
> +     take an exit before the IV exit.  */
> +  if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> +    bias_for_lowest = 1;
> +
>    int bias_for_assumed = bias_for_lowest;
>    int alignment_npeels = LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo);
>    if (alignment_npeels && LOOP_VINFO_USING_PARTIAL_VECTORS_P (loop_vinfo))
> 
> 
> 
> 
>

Richard Biener <rguenther@suse.de> writes:
> On Fri, 12 Apr 2024, Tamar Christina wrote:
>
>> Hi All,
>> 
>> This is a story all about how the peeling for gaps introduces a bug in the upper
>> bounds.
>> 
>> Before I go further, I'll first explain how I understand this to work for loops
>> with a single exit.
>> 
>> When peeling for gaps we peel N < VF iterations to scalar.
>> This happens by removing N iterations from the calculation of niters such that
>> vect_iters * VF == niters is always false.
>> 
>> In other words, when we exit the vector loop we always fall to the scalar loop.
>> The loop bounds adjustment guarantees this. Because of this we potentially
>> execute a vector loop iteration less.  That is, if you're at the boundary
>> condition where niters % VF by peeling one or more scalar iterations the vector
>> loop executes one less.
>> 
>> This is accounted for by the adjustments in vect_transform_loops.  This
>> adjustment happens differently based on whether the the vector loop can be
>> partial or not:
>> 
>> Peeling for gaps sets the bias to 0 and then:
>> 
>> when not partial:  we take the floor of (scalar_upper_bound / VF) - 1 to get the
>> 		   vector latch iteration count.
>> 
>> when loop is partial:  For a single exit this means the loop is masked, we take
>>                        the ceil to account for the fact that the loop can handle
>> 		       the final partial iteration using masking.
>> 
>> Note that there's no difference between ceil an floor on the boundary condition.
>> There is a difference however when you're slightly above it. i.e. if scalar
>> iterates 14 times and VF = 4 and we peel 1 iteration for gaps.
>> 
>> The partial loop does ((13 + 0) / 4) - 1 == 2 vector iterations. and in effect
>> the partial iteration is ignored and it's done as scalar.
>> 
>> This is fine because the niters modification has capped the vector iteration at
>> 2.  So that when we reduce the induction values you end up entering the scalar
>> code with ind_var.2 = ind_var.1 + 2 * VF.
>> 
>> Now lets look at early breaks.  To make it esier I'll focus on the specific
>> testcase:
>> 
>> char buffer[64];
>> 
>> __attribute__ ((noipa))
>> buff_t *copy (buff_t *first, buff_t *last)
>> {
>>   char *buffer_ptr = buffer;
>>   char *const buffer_end = &buffer[SZ-1];
>>   int store_size = sizeof(first->Val);
>>   while (first != last && (buffer_ptr + store_size) <= buffer_end)
>>     {
>>       const char *value_data = (const char *)(&first->Val);
>>       __builtin_memcpy(buffer_ptr, value_data, store_size);
>>       buffer_ptr += store_size;
>>       ++first;
>>     }
>> 
>>   if (first == last)
>>     return 0;
>> 
>>   return first;
>> }
>> 
>> Here the first, early exit is on the condition:
>> 
>>   (buffer_ptr + store_size) <= buffer_end
>> 
>> and the main exit is on condition:
>> 
>>   first != last
>> 
>> This is important, as this bug only manifests itself when the first exit has a
>> known constant iteration count that's lower than the latch exit count.
>> 
>> because buffer holds 64 bytes, and VF = 4, unroll = 2, we end up processing 16
>> bytes per iteration.  So the exit has a known bounds of 8 + 1.
>> 
>> The vectorizer correctly analizes this:
>> 
>> Statement (exit)if (ivtmp_21 != 0)
>>  is executed at most 8 (bounded by 8) + 1 times in loop 1.
>> 
>> and as a consequence the IV is bound by 9:
>> 
>>   # vect_vec_iv_.14_117 = PHI <_118(9), { 9, 8, 7, 6 }(20)>
>>   ...
>>   vect_ivtmp_21.16_124 = vect_vec_iv_.14_117 + { 18446744073709551615, 18446744073709551615, 18446744073709551615, 18446744073709551615 };
>>   mask_patt_22.17_126 = vect_ivtmp_21.16_124 != { 0, 0, 0, 0 };
>>   if (mask_patt_22.17_126 == { -1, -1, -1, -1 })
>>     goto <bb 3>; [88.89%]
>>   else
>>     goto <bb 30>; [11.11%]
>> 
>> The imporant bits are this:
>> 
>> In this example the value of last - first = 416.
>> 
>> the calculated vector iteration count, is:
>> 
>>     x = (((ptr2 - ptr1) - 16) / 16) + 1 = 27
>> 
>> the bounds generated, adjusting for gaps:
>> 
>>    x == (((x - 1) >> 2) << 2)
>> 
>> which means we'll always fall through to the scalar code. as intended.
>> 
>> Here are two key things to note:
>> 
>> 1. In this loop, the early exit will always be the one taken.  When it's taken
>>    we enter the scalar loop with the correct induction value to apply the gap
>>    peeling.
>> 
>> 2. If the main exit is taken, the induction values assumes you've finished all
>>    vector iterations.  i.e. it assumes you have completed 24 iterations, as we
>>    treat the main exit the same for normal loop vect and early break when not
>>    PEELED.
>>    This means the induction value is adjusted to ind_var.2 = ind_var.1 + 24 * VF;
>> 
>> So what's going wrong.  The vectorizer's codegen is correct and efficient,
>> however when we adjust the upper bounds, that code knows that the loops upper
>> bound is based on the early exit. i.e. 8 latch iterations. or in other words.
>> It thinks the loop iterates once.
>> 
>> This is incorrect as the vector loop iterates twice, as it has set up the
>> induction value such that it exits at the early exit.   So it in effect iterates
>> 2.5x times.
>> 
>> Becuase the upper bound is incorrect, when we unroll it now exits from the main
>> exit which uses the incorrect induction value.
>> 
>> So there are three ways to fix this:
>> 
>> 1.  If we take the position that the main exit should support both premature
>>     exits and final exits then vect_update_ivs_after_vectorizer needs to be
>>     skipped for this case, and vectorizable_induction updated with  third case
>>     where we reduce with LAST reduction based on the IVs instead of assuming
>>     you're at the end of the vector loop.
>> 
>>     I don't like this approach.  It don't think we should add a third induction
>>     style to cover up an issue introduced by unrolling.  It makes the code
>>     harder to follow and makes main exits harder to reason about.
>> 
>> 2. We could say that vec_init_loop_exit_info should pick the exit which has the
>>    smallest known iteration count.  This would turn this case into a PEELED case
>>    and the induction values would be correct as we'd always recalculate them
>>    from a reduction.  This is suboptimal though as the reason we pick the latch
>>    exit as the IV one is to prevent having to rotate the loop.  This results
>>    in more efficient code for what we assume is the common case, i.e. the main
>>    exit.
>> 
>> 3. In PR113734 we've established that for vectorization of early breaks that we
>>    must always treat the loop as partial.  Here partiallity means that we have
>>    enough vector elements to start the iteration, but we may take an early exit
>>    and so never reach the latch/main exit.
>> 
>>    This requirement is overwritten by the peeling for gaps adjustment of the
>>    upper bound.  I believe the bug is simply that this shouldn't be done.
>>    The adjustment here is to indicate that the main exit always leads to the
>>    scalar loop when peeling for gaps.
>> 
>>    But this invariant is already always true for all early exits.  Remember that
>>    early exits restart the scalar loop at the start of the vector iteration, so
>>    the induction values will start it where we want to do the gaps peeling.
>> 
>> I think no# 3 is the correct fix, and also one that doesn't degrade code quality.
>> 
>> Note: I used memcpy and memcmp in the testcase, I'm not sure if I can rely on
>>       these being inlined? but I also don't know how to test for library support.
>> 
>> Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu and
>> no issues.
>> 
>> Ok for master?
>
> I agree with the patch outcome.  I think the reasoning is simpler
> because peeling for gaps applies to the scalar iteration IV but
> with multiple exits an upper bound can effectively apply to a
> different exit and thus a different "IV" we cannot simply adjust
> an existing upper bound as if it were a bound for the scalar iteration IV.
>
> So I think we should word it like the following.  Can you pick that
> up and also add the other testcase I produced?
>
> OK with those changes.
>
> Thanks,
> Richard.
>
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index 3ffcac8c613..c728856079b 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -12152,14 +12152,14 @@ vect_transform_loop (loop_vec_info loop_vinfo, 
> gimple 
> *loop_vectorized_call)
>    bool final_iter_may_be_partial
>      = LOOP_VINFO_USING_PARTIAL_VECTORS_P (loop_vinfo)
>        || LOOP_VINFO_EARLY_BREAKS (loop_vinfo);
> -  /* The minimum number of iterations performed by the epilogue.  This
> -     is 1 when peeling for gaps because we always need a final scalar
> -     iteration.  */
> -  int min_epilogue_iters = LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo) ? 1 : 
> 0;
> -  /* +1 to convert latch counts to loop iteration counts,
> -     -min_epilogue_iters to remove iterations that cannot be performed
> -       by the vector code.  */
> -  int bias_for_lowest = 1 - min_epilogue_iters;
> +  /* +1 to convert latch counts to loop iteration counts.  */
> +  int bias_for_lowest = 1;
> +  /* When we are peeling for gaps then we take away one scalar iteration
> +     from the vector loop.  Thus we can adjust the upper bound by one
> +     scalar iteration.  But only when we know the bound applies to the
> +     IV exit test which might not be true when we have multiple exits.  
> */
> +  if (! LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> +    bias_for_lowest -= LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo) ? 1 : 0;
>    int bias_for_assumed = bias_for_lowest;
>    int alignment_npeels = LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo);
>    if (alignment_npeels && LOOP_VINFO_USING_PARTIAL_VECTORS_P 
> (loop_vinfo))

Ah, nice.

As the person guilty of writing the current form of the code, I think
the spltting out of min_epilogue_iters was anticipating some future
feature (that never materialised) in which the value might not be
binary.  I agree that it's no longer worth keeping it separate after
this adjustment, since it's not clear how far it would generalise
to values greater than 1.

And yeah, in the irc discussion, I hadn't realised that the "early"
and "IV" exits had been rotated.

Thanks,
Richard