diff mbox

PR64182: Fix rounding division and modulus

Message ID 87oar898g9.fsf@e105548-lin.cambridge.arm.com
State New
Headers show

Commit Message

Richard Sandiford Dec. 12, 2014, 3:47 p.m. UTC 
Richard Biener <richard.guenther@gmail.com> writes:
> On Thu, Dec 11, 2014 at 1:26 PM, Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>> As pointed out in PR 64182, wide-int rounded division gets the
>> ties-away-from-zero case wrong for odd-numbered dividends, while
>> double_int gets the unsigned case wrong by unconditionally treating
>> a dividend or remainder with the top bit set as negative.  As Jakub
>> says, the test used in double_int might also have overflow problems.
>>
>> This patch uses:
>>
>>    abs (remainder) >= abs (dividend) - abs (remainder)
>>
>> for both wide-int and double_int and fixes the unsigned case in double_int.
>> I didn't know how to test the double_int change using input code so
>> resorted to doing some double_int arithmetic at the start of main.
>>
>> Thanks to Joseph for the testcase.
>>
>> Tested on x86_64-linux-gnu.  OK to install?
>
> Can you add a testcase?  You can follow the gcc.dg/plugin/sreal_plugin.c
> example, maybe even make it a generic host_test_plugin.c with separate
> files containing the actual tests.
>
> Otherwise ok.

Ah, hadn't realised we could do that.  Much neater than changing main :-)

Here's what I committed after retesting on x86_64-linux-gnu.  As well as
the testcase, I changed x - y to the more general wi::sub (x, y).

Thanks,
Richard


gcc/
	PR middle-end/64182
	* wide-int.h (wi::div_round, wi::mod_round): Fix rounding of tied
	cases.
	* double-int.c (div_and_round_double): Fix handling of unsigned
	cases.  Use same rounding approach as wide-int.h.

gcc/testsuite/
2014-xx-xx  Richard Sandiford  <richard.sandiford@arm.com>
	    Joseph Myers  <joseph@codesourcery.com>

	PR middle-end/64182
	* gcc.dg/plugin/wide-int-test-1.c,
	gcc.dg/plugin/wide-int_plugin.c: New test.
	* gcc.dg/plugin/plugin.exp: Register it.
	* gnat.dg/round_div.adb: New test.
diff mbox

Patch

Index: gcc/wide-int.h
===================================================================
--- gcc/wide-int.h	2014-12-11 14:32:17.708138315 +0000
+++ gcc/wide-int.h	2014-12-11 14:32:17.704138366 +0000
@@ -2616,8 +2616,8 @@  wi::div_round (const T1 &x, const T2 &y,
     {
       if (sgn == SIGNED)
 	{
-	  if (wi::ges_p (wi::abs (remainder),
-			 wi::lrshift (wi::abs (y), 1)))
+	  WI_BINARY_RESULT (T1, T2) abs_remainder = wi::abs (remainder);
+	  if (wi::geu_p (abs_remainder, wi::sub (wi::abs (y), abs_remainder)))
 	    {
 	      if (wi::neg_p (x, sgn) != wi::neg_p (y, sgn))
 		return quotient - 1;
@@ -2627,7 +2627,7 @@  wi::div_round (const T1 &x, const T2 &y,
 	}
       else
 	{
-	  if (wi::geu_p (remainder, wi::lrshift (y, 1)))
+	  if (wi::geu_p (remainder, wi::sub (y, remainder)))
 	    return quotient + 1;
 	}
     }
@@ -2784,8 +2784,8 @@  wi::mod_round (const T1 &x, const T2 &y,
     {
       if (sgn == SIGNED)
 	{
-	  if (wi::ges_p (wi::abs (remainder),
-			 wi::lrshift (wi::abs (y), 1)))
+	  WI_BINARY_RESULT (T1, T2) abs_remainder = wi::abs (remainder);
+	  if (wi::geu_p (abs_remainder, wi::sub (wi::abs (y), abs_remainder)))
 	    {
 	      if (wi::neg_p (x, sgn) != wi::neg_p (y, sgn))
 		return remainder + y;
@@ -2795,7 +2795,7 @@  wi::mod_round (const T1 &x, const T2 &y,
 	}
       else
 	{
-	  if (wi::geu_p (remainder, wi::lrshift (y, 1)))
+	  if (wi::geu_p (remainder, wi::sub (y, remainder)))
 	    return remainder - y;
 	}
     }
Index: gcc/double-int.c
===================================================================
--- gcc/double-int.c	2014-12-11 14:32:17.708138315 +0000
+++ gcc/double-int.c	2014-12-11 14:32:17.700138416 +0000
@@ -569,24 +569,23 @@  div_and_round_double (unsigned code, int
       {
 	unsigned HOST_WIDE_INT labs_rem = *lrem;
 	HOST_WIDE_INT habs_rem = *hrem;
-	unsigned HOST_WIDE_INT labs_den = lden, ltwice;
-	HOST_WIDE_INT habs_den = hden, htwice;
+	unsigned HOST_WIDE_INT labs_den = lden, lnegabs_rem, ldiff;
+	HOST_WIDE_INT habs_den = hden, hnegabs_rem, hdiff;
 
 	/* Get absolute values.  */
-	if (*hrem < 0)
+	if (!uns && *hrem < 0)
 	  neg_double (*lrem, *hrem, &labs_rem, &habs_rem);
-	if (hden < 0)
+	if (!uns && hden < 0)
 	  neg_double (lden, hden, &labs_den, &habs_den);
 
-	/* If (2 * abs (lrem) >= abs (lden)), adjust the quotient.  */
-	mul_double ((HOST_WIDE_INT) 2, (HOST_WIDE_INT) 0,
-		    labs_rem, habs_rem, &ltwice, &htwice);
+	/* If abs(rem) >= abs(den) - abs(rem), adjust the quotient.  */
+	neg_double (labs_rem, habs_rem, &lnegabs_rem, &hnegabs_rem);
+	add_double (labs_den, habs_den, lnegabs_rem, hnegabs_rem,
+		    &ldiff, &hdiff);
 
-	if (((unsigned HOST_WIDE_INT) habs_den
-	     < (unsigned HOST_WIDE_INT) htwice)
-	    || (((unsigned HOST_WIDE_INT) habs_den
-		 == (unsigned HOST_WIDE_INT) htwice)
-		&& (labs_den <= ltwice)))
+	if (((unsigned HOST_WIDE_INT) habs_rem
+	     > (unsigned HOST_WIDE_INT) hdiff)
+	    || (habs_rem == hdiff && labs_rem >= ldiff))
 	  {
 	    if (quo_neg)
 	      /* quo = quo - 1;  */
Index: gcc/testsuite/gcc.dg/plugin/wide-int-test-1.c
===================================================================
--- /dev/null	2014-11-19 08:41:51.310561007 +0000
+++ gcc/testsuite/gcc.dg/plugin/wide-int-test-1.c	2014-12-11 14:32:17.704138366 +0000
@@ -0,0 +1,9 @@ 
+/* Test that pass is inserted and invoked once. */
+/* { dg-do compile } */
+/* { dg-options "-O" } */
+
+int
+main (int argc, char **argv)
+{
+  return 0;
+}
Index: gcc/testsuite/gcc.dg/plugin/wide-int_plugin.c
===================================================================
--- /dev/null	2014-11-19 08:41:51.310561007 +0000
+++ gcc/testsuite/gcc.dg/plugin/wide-int_plugin.c	2014-12-11 14:32:27.792013373 +0000
@@ -0,0 +1,46 @@ 
+#include "config.h"
+#include "gcc-plugin.h"
+#include "system.h"
+#include "coretypes.h"
+#include "tree.h"
+
+int plugin_is_GPL_compatible;
+
+static void
+test_double_int_round_udiv (void)
+{
+  double_int dmin = { 0, HOST_WIDE_INT_MIN };
+  double_int dmax = { -1, HOST_WIDE_INT_MAX };
+  double_int dnegone = { -1, -1 };
+  double_int mod, div;
+  div = dmin.udivmod (dnegone, ROUND_DIV_EXPR, &mod);
+  if (div.low != 1 || div.high != 0
+      || mod.low != 1 || mod.high != HOST_WIDE_INT_MIN)
+    abort ();
+  div = dmax.udivmod (dnegone, ROUND_DIV_EXPR, &mod);
+  if (div.low != 0 || div.high != 0
+      || mod.low != dmax.low || mod.high != dmax.high)
+    abort ();
+}
+
+static void
+test_wide_int_round_sdiv (void)
+{
+  if (wi::ne_p (wi::div_round (2, 3, SIGNED), 1))
+    abort ();
+  if (wi::ne_p (wi::div_round (1, 3, SIGNED), 0))
+    abort ();
+  if (wi::ne_p (wi::mod_round (2, 3, SIGNED), -1))
+    abort ();
+  if (wi::ne_p (wi::mod_round (1, 3, SIGNED), 1))
+    abort ();
+}
+
+int
+plugin_init (struct plugin_name_args *plugin_info,
+	     struct plugin_gcc_version *version)
+{
+  test_double_int_round_udiv ();
+  test_wide_int_round_sdiv ();
+  return 0;
+}
Index: gcc/testsuite/gcc.dg/plugin/plugin.exp
===================================================================
--- gcc/testsuite/gcc.dg/plugin/plugin.exp	2014-12-11 14:32:17.708138315 +0000
+++ gcc/testsuite/gcc.dg/plugin/plugin.exp	2014-12-11 14:32:17.704138366 +0000
@@ -62,6 +62,7 @@  set plugin_test_list [list \
     { sreal_plugin.c sreal-test-1.c } \
     { start_unit_plugin.c start_unit-test-1.c } \
     { finish_unit_plugin.c finish_unit-test-1.c } \
+    { wide-int_plugin.c wide-int-test-1.c } \
 ]
 
 foreach plugin_test $plugin_test_list {
Index: gcc/testsuite/gnat.dg/round_div.adb
===================================================================
--- /dev/null	2014-11-19 08:41:51.310561007 +0000
+++ gcc/testsuite/gnat.dg/round_div.adb	2014-12-11 14:32:17.700138416 +0000
@@ -0,0 +1,17 @@ 
+-- { dg-do run }
+-- { dg-options "-O3" }
+procedure Round_Div is
+   type Fixed is delta 1.0 range -2147483648.0 .. 2147483647.0;
+   A : Fixed := 1.0;
+   B : Fixed := 3.0;
+   C : Integer;
+   function Divide (X, Y : Fixed) return Integer is
+   begin
+      return Integer (X / Y);
+   end;
+begin
+   C := Divide (A, B);
+   if C /= 0 then
+      raise Program_Error;
+   end if;
+end Round_Div;