diff mbox series

[1/2,middle-end/102276] Don't emit switch-unreachable warnings for -ftrivial-auto-var-init (PR102276)

Message ID 4BFFFF31-FDD2-4145-BC6F-B7CBB925B1DB@oracle.com
State New
Headers show
Series [1/2,middle-end/102276] Don't emit switch-unreachable warnings for -ftrivial-auto-var-init (PR102276) | expand

Commit Message

Qing Zhao Feb. 19, 2022, 4:22 p.m. UTC 
Hi,

Per our discussion in the bug report https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102276

We decided to go with the following solution:

1. avoid emitting switch-unreachable warnings for -ftrivial-auto-var-init;
2. adding a new option -Wtrivial-auto-var-init to emit warnings for the switch-unreadable cases to suggest the user modify the source code;
3. update documentation of -ftrivial-auto-var-init for the limitation on switch-unreachable cases and introduce the new option -Wtrivial-auto-var-init

with the above 1, we can resolve the current immediate issue of spurious warnings of using -ftrivial-auto-var-init to make kernel build succeed;
with the above 2, we provide the user a way to know that -ftrivial-auto-var-init has limitation on the switch-unreachable cases, and user should modify the source code to avoid this problem;
with the above 3, we will provide the user a clear documentation of the -ftrivial-auto-var-init and also provide suggestions how to resolve this issue. 

There are two patches included for this bug.  This is the first one.

The patches has been bootstrapped and regression tested on both x86 and aarch64.

Okay for commit?

Thanks.

Qing.

===========================

From 65bc9607ff35ad49e5501ec5c392293c5b6358d0 Mon Sep 17 00:00:00 2001
From: Qing Zhao <qing.zhao@oracle.com>
Date: Fri, 18 Feb 2022 15:35:53 +0000
Subject: [PATCH 1/2] Don't emit switch-unreachable warnings for
 -ftrivial-auto-var-init (PR102276)

for the following testing case:
  1 int g(int *);
  2 int f1()
  3 {
  4     switch (0) {
  5         int x;
  6         default:
  7         return g(&x);
  8     }
  9 }
compiling with -O -ftrivial-auto-var-init causes spurious warning:
warning: statement will never be executed [-Wswitch-unreachable]
    5 |         int x;
      |             ^
This is due to the compiler-generated initialization at the point of
the declaration.

We could avoid the warning by adjusting the routine
"maybe_warn_switch_unreachable" to exclude the following cases:

when
flag_auto_var_init > AUTO_INIT_UNINITIALIZED
And
call to .DEFERRED_INIT

2022-02-18 Qing Zhao  <qing.zhao@oracle.com>
gcc/ChangeLog:

	* gimplify.cc (maybe_warn_switch_unreachable): Don't warn for compiler
	-generated initializations for -ftrivial-auto-var-init.

gcc/testsuite/ChangeLog:

	* gcc.dg/auto-init-pr102276-1.c: New test.
	* gcc.dg/auto-init-pr102276-2.c: New test.
---
 gcc/gimplify.cc                             |  8 ++++-
 gcc/testsuite/gcc.dg/auto-init-pr102276-1.c | 38 +++++++++++++++++++++
 gcc/testsuite/gcc.dg/auto-init-pr102276-2.c | 38 +++++++++++++++++++++
 3 files changed, 83 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/auto-init-pr102276-1.c
 create mode 100644 gcc/testsuite/gcc.dg/auto-init-pr102276-2.c

Comments

Richard Biener Feb. 24, 2022, 10:10 a.m. UTC | #1 
On Sat, 19 Feb 2022, Qing Zhao wrote:

> Hi,
> 
> Per our discussion in the bug report https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102276
> 
> We decided to go with the following solution:
> 
> 1. avoid emitting switch-unreachable warnings for -ftrivial-auto-var-init;
> 2. adding a new option -Wtrivial-auto-var-init to emit warnings for the switch-unreadable cases to suggest the user modify the source code;
> 3. update documentation of -ftrivial-auto-var-init for the limitation on switch-unreachable cases and introduce the new option -Wtrivial-auto-var-init
> 
> with the above 1, we can resolve the current immediate issue of spurious warnings of using -ftrivial-auto-var-init to make kernel build succeed;
> with the above 2, we provide the user a way to know that -ftrivial-auto-var-init has limitation on the switch-unreachable cases, and user should modify the source code to avoid this problem;
> with the above 3, we will provide the user a clear documentation of the -ftrivial-auto-var-init and also provide suggestions how to resolve this issue. 
> 
> There are two patches included for this bug.  This is the first one.
> 
> The patches has been bootstrapped and regression tested on both x86 and aarch64.
> 
> Okay for commit?
> 
> Thanks.
> 
> Qing.
> 
> ===========================
> 
> From 65bc9607ff35ad49e5501ec5c392293c5b6358d0 Mon Sep 17 00:00:00 2001
> From: Qing Zhao <qing.zhao@oracle.com>
> Date: Fri, 18 Feb 2022 15:35:53 +0000
> Subject: [PATCH 1/2] Don't emit switch-unreachable warnings for
>  -ftrivial-auto-var-init (PR102276)
> 
> for the following testing case:
>   1 int g(int *);
>   2 int f1()
>   3 {
>   4     switch (0) {
>   5         int x;
>   6         default:
>   7         return g(&x);
>   8     }
>   9 }
> compiling with -O -ftrivial-auto-var-init causes spurious warning:
> warning: statement will never be executed [-Wswitch-unreachable]
>     5 |         int x;
>       |             ^
> This is due to the compiler-generated initialization at the point of
> the declaration.
> 
> We could avoid the warning by adjusting the routine
> "maybe_warn_switch_unreachable" to exclude the following cases:
> 
> when
> flag_auto_var_init > AUTO_INIT_UNINITIALIZED
> And
> call to .DEFERRED_INIT
> 
> 2022-02-18 Qing Zhao  <qing.zhao@oracle.com>
> gcc/ChangeLog:
> 
> 	* gimplify.cc (maybe_warn_switch_unreachable): Don't warn for compiler
> 	-generated initializations for -ftrivial-auto-var-init.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.dg/auto-init-pr102276-1.c: New test.
> 	* gcc.dg/auto-init-pr102276-2.c: New test.
> ---
>  gcc/gimplify.cc                             |  8 ++++-
>  gcc/testsuite/gcc.dg/auto-init-pr102276-1.c | 38 +++++++++++++++++++++
>  gcc/testsuite/gcc.dg/auto-init-pr102276-2.c | 38 +++++++++++++++++++++
>  3 files changed, 83 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.dg/auto-init-pr102276-1.c
>  create mode 100644 gcc/testsuite/gcc.dg/auto-init-pr102276-2.c
> 
> diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
> index f570daa015a..4e3bbf5314d 100644
> --- a/gcc/gimplify.cc
> +++ b/gcc/gimplify.cc
> @@ -2103,7 +2103,13 @@ maybe_warn_switch_unreachable (gimple_seq seq)
>  	  && TREE_CODE (gimple_goto_dest (stmt)) == LABEL_DECL
>  	  && DECL_ARTIFICIAL (gimple_goto_dest (stmt)))
>  	/* Don't warn for compiler-generated gotos.  These occur
> -	   in Duff's devices, for example.  */;
> +	   in Duff's devices, for example.  */
> +	;
> +      else if ((flag_auto_var_init > AUTO_INIT_UNINITIALIZED)
> +		&& (gimple_call_internal_p (stmt, IFN_DEFERRED_INIT)))
> +	/* Don't warn for compiler-generated initializations for
> +	  -ftrivial-auto-var-init.  */
> +	;

I think you want to instead skip these in warn_switch_unreachable_r
since otherwise a .DEFERRED_INIT can silence the warning for a real
stmt following it that is not reachable.

Richard.

>        else
>  	warning_at (gimple_location (stmt), OPT_Wswitch_unreachable,
>  		    "statement will never be executed");
> diff --git a/gcc/testsuite/gcc.dg/auto-init-pr102276-1.c b/gcc/testsuite/gcc.dg/auto-init-pr102276-1.c
> new file mode 100644
> index 00000000000..d574926e0c8
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/auto-init-pr102276-1.c
> @@ -0,0 +1,38 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -Wall -ftrivial-auto-var-init=zero" } */
> +
> +int g(int *);
> +int f()
> +{
> +    switch (0) { 
> +        int x;  /* { dg-bogus "statement will never be executed" } */
> +        default:
> +        return g(&x);
> +    }
> +}
> +
> +int g1(int);
> +int f1()
> +{
> +    switch (0) {
> +        int x; /* { dg-bogus "statement will never be executed" } */
> +        default:
> +        return g1(x);  /* { dg-warning "is used uninitialized" } */
> +    }
> +}
> +
> +struct S
> +{
> +  char a;
> +  int b;
> +};
> +int g2(int);
> +int f2(int input)
> +{
> +    switch (0) {
> +        struct S x; /* { dg-bogus "statement will never be executed" } */
> +        default:
> +        return g2(input) + x.b;  /* { dg-warning "is used uninitialized" } */
> +    }
> +}
> +
> diff --git a/gcc/testsuite/gcc.dg/auto-init-pr102276-2.c b/gcc/testsuite/gcc.dg/auto-init-pr102276-2.c
> new file mode 100644
> index 00000000000..779d3ec3882
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/auto-init-pr102276-2.c
> @@ -0,0 +1,38 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -Wall -ftrivial-auto-var-init=pattern" } */
> +
> +int g(int *);
> +int f()
> +{
> +    switch (0) { 
> +        int x;  /* { dg-bogus "statement will never be executed" } */
> +        default:
> +        return g(&x);
> +    }
> +}
> +
> +int g1(int);
> +int f1()
> +{
> +    switch (0) {
> +        int x; /* { dg-bogus "statement will never be executed" } */
> +        default:
> +        return g1(x);  /* { dg-warning "is used uninitialized" } */
> +    }
> +}
> +
> +struct S
> +{
> +  char a;
> +  int b;
> +};
> +int g2(int);
> +int f2(int input)
> +{
> +    switch (0) {
> +        struct S x; /* { dg-bogus "statement will never be executed" } */
> +        default:
> +        return g2(input) + x.b;  /* { dg-warning "is used uninitialized" } */
> +    }
> +}
> +
>
Qing Zhao Feb. 24, 2022, 3:14 p.m. UTC | #2 
> On Feb 24, 2022, at 4:10 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Sat, 19 Feb 2022, Qing Zhao wrote:
> 
>> Hi,
>> 
>> Per our discussion in the bug report https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102276
>> 
>> We decided to go with the following solution:
>> 
>> 1. avoid emitting switch-unreachable warnings for -ftrivial-auto-var-init;
>> 2. adding a new option -Wtrivial-auto-var-init to emit warnings for the switch-unreadable cases to suggest the user modify the source code;
>> 3. update documentation of -ftrivial-auto-var-init for the limitation on switch-unreachable cases and introduce the new option -Wtrivial-auto-var-init
>> 
>> with the above 1, we can resolve the current immediate issue of spurious warnings of using -ftrivial-auto-var-init to make kernel build succeed;
>> with the above 2, we provide the user a way to know that -ftrivial-auto-var-init has limitation on the switch-unreachable cases, and user should modify the source code to avoid this problem;
>> with the above 3, we will provide the user a clear documentation of the -ftrivial-auto-var-init and also provide suggestions how to resolve this issue. 
>> 
>> There are two patches included for this bug.  This is the first one.
>> 
>> The patches has been bootstrapped and regression tested on both x86 and aarch64.
>> 
>> Okay for commit?
>> 
>> Thanks.
>> 
>> Qing.
>> 
>> ===========================
>> 
>> From 65bc9607ff35ad49e5501ec5c392293c5b6358d0 Mon Sep 17 00:00:00 2001
>> From: Qing Zhao <qing.zhao@oracle.com>
>> Date: Fri, 18 Feb 2022 15:35:53 +0000
>> Subject: [PATCH 1/2] Don't emit switch-unreachable warnings for
>> -ftrivial-auto-var-init (PR102276)
>> 
>> for the following testing case:
>>  1 int g(int *);
>>  2 int f1()
>>  3 {
>>  4     switch (0) {
>>  5         int x;
>>  6         default:
>>  7         return g(&x);
>>  8     }
>>  9 }
>> compiling with -O -ftrivial-auto-var-init causes spurious warning:
>> warning: statement will never be executed [-Wswitch-unreachable]
>>    5 |         int x;
>>      |             ^
>> This is due to the compiler-generated initialization at the point of
>> the declaration.
>> 
>> We could avoid the warning by adjusting the routine
>> "maybe_warn_switch_unreachable" to exclude the following cases:
>> 
>> when
>> flag_auto_var_init > AUTO_INIT_UNINITIALIZED
>> And
>> call to .DEFERRED_INIT
>> 
>> 2022-02-18 Qing Zhao  <qing.zhao@oracle.com>
>> gcc/ChangeLog:
>> 
>> 	* gimplify.cc (maybe_warn_switch_unreachable): Don't warn for compiler
>> 	-generated initializations for -ftrivial-auto-var-init.
>> 
>> gcc/testsuite/ChangeLog:
>> 
>> 	* gcc.dg/auto-init-pr102276-1.c: New test.
>> 	* gcc.dg/auto-init-pr102276-2.c: New test.
>> ---
>> gcc/gimplify.cc                             |  8 ++++-
>> gcc/testsuite/gcc.dg/auto-init-pr102276-1.c | 38 +++++++++++++++++++++
>> gcc/testsuite/gcc.dg/auto-init-pr102276-2.c | 38 +++++++++++++++++++++
>> 3 files changed, 83 insertions(+), 1 deletion(-)
>> create mode 100644 gcc/testsuite/gcc.dg/auto-init-pr102276-1.c
>> create mode 100644 gcc/testsuite/gcc.dg/auto-init-pr102276-2.c
>> 
>> diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
>> index f570daa015a..4e3bbf5314d 100644
>> --- a/gcc/gimplify.cc
>> +++ b/gcc/gimplify.cc
>> @@ -2103,7 +2103,13 @@ maybe_warn_switch_unreachable (gimple_seq seq)
>> 	  && TREE_CODE (gimple_goto_dest (stmt)) == LABEL_DECL
>> 	  && DECL_ARTIFICIAL (gimple_goto_dest (stmt)))
>> 	/* Don't warn for compiler-generated gotos.  These occur
>> -	   in Duff's devices, for example.  */;
>> +	   in Duff's devices, for example.  */
>> +	;
>> +      else if ((flag_auto_var_init > AUTO_INIT_UNINITIALIZED)
>> +		&& (gimple_call_internal_p (stmt, IFN_DEFERRED_INIT)))
>> +	/* Don't warn for compiler-generated initializations for
>> +	  -ftrivial-auto-var-init.  */
>> +	;
> 
> I think you want to instead skip these in warn_switch_unreachable_r
> since otherwise a .DEFERRED_INIT can silence the warning for a real
> stmt following it that is not reachable.

Oh, yeah, you are right.
Will fix this.

Thanks.

Qing
diff mbox series

Patch

diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index f570daa015a..4e3bbf5314d 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -2103,7 +2103,13 @@  maybe_warn_switch_unreachable (gimple_seq seq)
 	  && TREE_CODE (gimple_goto_dest (stmt)) == LABEL_DECL
 	  && DECL_ARTIFICIAL (gimple_goto_dest (stmt)))
 	/* Don't warn for compiler-generated gotos.  These occur
-	   in Duff's devices, for example.  */;
+	   in Duff's devices, for example.  */
+	;
+      else if ((flag_auto_var_init > AUTO_INIT_UNINITIALIZED)
+		&& (gimple_call_internal_p (stmt, IFN_DEFERRED_INIT)))
+	/* Don't warn for compiler-generated initializations for
+	  -ftrivial-auto-var-init.  */
+	;
       else
 	warning_at (gimple_location (stmt), OPT_Wswitch_unreachable,
 		    "statement will never be executed");
diff --git a/gcc/testsuite/gcc.dg/auto-init-pr102276-1.c b/gcc/testsuite/gcc.dg/auto-init-pr102276-1.c
new file mode 100644
index 00000000000..d574926e0c8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/auto-init-pr102276-1.c
@@ -0,0 +1,38 @@ 
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wall -ftrivial-auto-var-init=zero" } */
+
+int g(int *);
+int f()
+{
+    switch (0) { 
+        int x;  /* { dg-bogus "statement will never be executed" } */
+        default:
+        return g(&x);
+    }
+}
+
+int g1(int);
+int f1()
+{
+    switch (0) {
+        int x; /* { dg-bogus "statement will never be executed" } */
+        default:
+        return g1(x);  /* { dg-warning "is used uninitialized" } */
+    }
+}
+
+struct S
+{
+  char a;
+  int b;
+};
+int g2(int);
+int f2(int input)
+{
+    switch (0) {
+        struct S x; /* { dg-bogus "statement will never be executed" } */
+        default:
+        return g2(input) + x.b;  /* { dg-warning "is used uninitialized" } */
+    }
+}
+
diff --git a/gcc/testsuite/gcc.dg/auto-init-pr102276-2.c b/gcc/testsuite/gcc.dg/auto-init-pr102276-2.c
new file mode 100644
index 00000000000..779d3ec3882
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/auto-init-pr102276-2.c
@@ -0,0 +1,38 @@ 
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wall -ftrivial-auto-var-init=pattern" } */
+
+int g(int *);
+int f()
+{
+    switch (0) { 
+        int x;  /* { dg-bogus "statement will never be executed" } */
+        default:
+        return g(&x);
+    }
+}
+
+int g1(int);
+int f1()
+{
+    switch (0) {
+        int x; /* { dg-bogus "statement will never be executed" } */
+        default:
+        return g1(x);  /* { dg-warning "is used uninitialized" } */
+    }
+}
+
+struct S
+{
+  char a;
+  int b;
+};
+int g2(int);
+int f2(int input)
+{
+    switch (0) {
+        struct S x; /* { dg-bogus "statement will never be executed" } */
+        default:
+        return g2(input) + x.b;  /* { dg-warning "is used uninitialized" } */
+    }
+}
+