https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104574
Bug ID: 104574
Summary: GCC misses basic optimization for restricted pointers
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: m...@nieper-wisskirchen.de
Target Milestone: ---
The following TU
void g (void);
int
f (int *restrict p)
{
p[0] = 7;
g ();
return p[0];
}
is compiled by GCC at -O3 to
f:
pushq %rbx
movq %rdi, %rbx
movl $7, (%rdi)
call g
movl (%rbx), %eax
popq %rbx
ret
Obviously, GCC seems to think that g may modify the data reachable through p.
However, if g did that it would cause undefined behavior anyway.
So GCC misses a simple optimization opportunity here; it doesn't have to reload
the memory contents after the call to g; in fact, the function will always
return 7.
For comparison, Clang compiles the TU to
f:
pushq %rax
movl $7, (%rdi)
callq g
movl $7, %eax
popq %rcx
retq
