------- Comment #4 from rguenth at gcc dot gnu dot org 2010-02-15 13:16 -------
It also fails with -O1 and -O2 -fno-strict-aliasing and with -O2 -fno-inline.
Assembler differences for -O2 vs. -O2 with my patch (which effectively
makes us see more must-aliases, thus "accept" slightly invalid
strict-aliasing violating code):
--- ref2.s.good 2010-02-15 13:47:03.000000000 +0100
+++ ref2.s.bad 2010-02-15 13:46:34.000000000 +0100
@@ -90,15 +90,16 @@
.cfi_startproc
subq $40, %rsp
.cfi_def_cfa_offset 48
- leaq 28(%rsp), %rsi
+ leaq 28(%rsp), %rdx
movl $1, 28(%rsp)
movq $_ZNK3Inc1fERi, (%rsp)
movq $0, 8(%rsp)
leaq 16(%rsp), %rdi
- movq %rsi, 16(%rsp)
+ movq %rdx, 16(%rsp)
movb %al, 16(%rsp)
movl $_ZNK3Inc1fERi, %eax
testb $1, %al
+ movq 16(%rsp), %rsi
je .L10
movq _ZNK3Inc1fERi-1(%rsi), %rax
.L10:
that's _Z6test02v. You can see the aliasing byte-store to 16(%rsp) and
the probably seemingly redundant load from 16(%rsp) that we maybe remove
with strict-aliasing on.
The tree code for the above is at .optimize time:
<bb 2>:
counter = 1;
D.29754 = {};
__bound_args#0 = D.29754;
D.25693._M_f.__pmf.__pfn = f;
D.25693._M_f.__pmf.__delta = 0;
D.25693._M_bound_args.D.25507.D.25257.D.25050._M_head_impl._M_data =
&counter;
this.24_34 = (struct Inc *) &D.25693._M_bound_args.D.25507;
*this.24_34 = __bound_args#0;
D.29831_51 =
D.25693._M_bound_args.D.25507.D.25257.D.25050._M_head_impl._M_data;
iftmp.27_55 = D.25693._M_f.__pmf.__pfn;
D.29837_56 = (long int) iftmp.27_55;
D.29838_57 = D.29837_56 & 1;
if (D.29838_57 != 0)
goto <bb 3>;
else
goto <bb 4>;
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43075
