https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85315
Bug ID: 85315
Summary: missed optimisation opportunity for derefences
Product: gcc
Version: 8.0.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vegard.nossum at oracle dot com
Target Milestone: ---
Input:
extern int x;
extern int a;
extern int b;
int f()
{
int y = x;
return *(&y + (a + b));
}
With -O3, trunk outputs:
f():
movl x(%rip), %eax
movl %eax, -4(%rsp)
movl b(%rip), %eax
addl a(%rip), %eax
cltq
movl -4(%rsp,%rax,4), %eax
ret
Clang, on the other hand, infers that (a + b) == 0:
f(): # @f()
movl x(%rip), %eax
retq
From richi on IRC:
"""
we don't do this kind of optimization at the moment
a related one would be to place a if (a+b != 0) link_error (); after the memory
access
similarly for array accesses an if (i not-in-range) link_error ()
thus, we do not derive ranges for address-computation parts [at dereference
sites]
"""
