From: Jason Merrill <jason@redhat.com>
Date: Fri, 9 Jul 2021 09:45:03 +0000 (-0400)
Subject: c++: enable -fdelete-dead-exceptions by default
X-Git-Tag: basepoints/gcc-13~6006
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b15e301748f0e042379909e32b3ade439dd8f8f9;p=thirdparty%2Fgcc.git

c++: enable -fdelete-dead-exceptions by default

As I was discussing with richi, I don't think it makes sense to protect
calls to pure/const functions from DCE just because they aren't explicitly
declared noexcept.  PR100382 indicates that there are different
considerations for Go, which has non-call exceptions.  But still turn the
flag off for that specific testcase.

gcc/c-family/ChangeLog:

	* c-opts.c (c_common_post_options): Set -fdelete-dead-exceptions.

gcc/ChangeLog:

	* doc/invoke.texi: -fdelete-dead-exceptions is on by default for
	C++.

gcc/testsuite/ChangeLog:

	* g++.dg/torture/pr100382.C: Pass -fno-delete-dead-exceptions.
---

diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index 60b5802722ce..1c4e832c7ed0 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -1015,6 +1015,10 @@ c_common_post_options (const char **pfilename)
   SET_OPTION_IF_UNSET (&global_options, &global_options_set, flag_finite_loops,
 		       optimize >= 2 && cxx_dialect >= cxx11);
 
+  /* It's OK to discard calls to pure/const functions that might throw.  */
+  SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+		       flag_delete_dead_exceptions, true);
+
   if (cxx_dialect >= cxx11)
     {
       /* If we're allowing C++0x constructs, don't warn about C++98
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index e67d47af676d..ea8812425e95 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -16335,8 +16335,10 @@ arbitrary signal handlers such as @code{SIGALRM}.
 @opindex fdelete-dead-exceptions
 Consider that instructions that may throw exceptions but don't otherwise
 contribute to the execution of the program can be optimized away.
-This option is enabled by default for the Ada compiler, as permitted by
-the Ada language specification.
+This does not affect calls to functions except those with the
+@code{pure} or @code{const} attributes.
+This option is enabled by default for the Ada and C++ compilers, as permitted by
+the language specifications.
 Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.
 
 @item -funwind-tables
diff --git a/gcc/testsuite/g++.dg/torture/pr100382.C b/gcc/testsuite/g++.dg/torture/pr100382.C
index ffc4182cfcea..eac5743b956d 100644
--- a/gcc/testsuite/g++.dg/torture/pr100382.C
+++ b/gcc/testsuite/g++.dg/torture/pr100382.C
@@ -1,4 +1,5 @@
 // { dg-do run }
+// { dg-additional-options -fno-delete-dead-exceptions }
 
 int x, y;
 int __attribute__((pure,noinline)) foo () { if (x) throw 1; return y; }