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Why is -fstrict-aliasing excluded from function "optimize" attribute?
- From: "Bingfeng Mei" <bmei at broadcom dot com>
- To: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Wed, 3 Nov 2010 09:49:31 -0700
- Subject: Why is -fstrict-aliasing excluded from function "optimize" attribute?
Hello,
I came across an issue with function "optimize" attribute. The code is like:
__attribute__((optimize("-fno-strict-aliasing")))
void foo()
{
...
}
When compiling with -O2, we expect this function is compiled without following
strict aliasing rule, whereas other code does. However, I found this function still
has strict aliasing flag turned on during compilation. After a little investigation,
I found the following lines of code in parse_optimize_options of c-common.c
(4.5, 4.6 is similar)
...
saved_flag_strict_aliasing = flag_strict_aliasing;
/* Now parse the options. */
decode_options (opt_argc, opt_argv);
targetm.override_options_after_change();
/* Don't allow changing -fstrict-aliasing. */
flag_strict_aliasing = saved_flag_strict_aliasing;
...
-fstrict-aliasing is excluded from function specific optimize option. I checked
both internal manual and gcc manual. It seems not to be documented. I wonder
whether there is a good reason for this? If yes, at least we should document it.
BTW, the code certainly works in 4.4.
Regards,
Bingfeng Mei