https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113106
Bug ID: 113106
Summary: Missing CSE with cast to volatile
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: ubizjak at gmail dot com
Target Milestone: ---
The following testcase:
--cut here--
int a;
int foo(void)
{
return *(volatile int *) &a + a;
}
--cut here--
compiles with -O2 to:
movl a(%rip), %eax
addl a(%rip), %eax
ret
with more detail:
#(insn:TI 6 7 8 2 (set (reg:SI 0 ax [orig:98 _1 ] [98])
# (mem/v/c:SI (symbol_ref:DI ("a") [flags 0x2] <var_decl
0x7f59db15ac60a>) [1 MEM[(volatile int *)&a]+0 S4 A32])) "vol.c":5:10 85
{*movsi_internal}
# (nil))
movl a(%rip), %eax # 6 [c=5 l=6] *movsi_internal/0
#(insn 8 6 14 2 (parallel [
# (set (reg:SI 0 ax [102])
# (plus:SI (reg:SI 0 ax [orig:98 _1 ] [98])
# (mem/c:SI (symbol_ref:DI ("a") [flags 0x2] <var_decl
0x7f59db15ac60 a>) [1 a+0 S4 A32])))
# (clobber (reg:CC 17 flags))
# ]) "vol.c":5:31 271 {*addsi_1}
# (expr_list:REG_UNUSED (reg:CC 17 flags)
# (nil)))
addl a(%rip), %eax # 8 [c=9 l=6] *addsi_1/1
This may be compiled to:
movl a(%rip), %eax
addl %eax, %eax
ret
since only one read uses volatile.
