https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95078
Bug ID: 95078
Summary: Missing fwprop for SIB address
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: crazylht at gmail dot com
CC: hjl.tools at gmail dot com
Target Milestone: ---
Target: i386, x86-64
cat test.c
int foo (int* p1, int* p2, int scale)
{
int ret = *(p1 + scale * 4 + 11);
*p2 = 3;
int ret2 = *(p1 + scale * 4 + 11);
return ret + ret2;
}
gcc11 -O2 test.c -S
foo(int*, int*, int):
sall $2, %edx
movslq %edx, %rdx
leaq 44(%rdi,%rdx,4), %rdx --- redundant could be fwprop
movl (%rdx), %eax
movl $3, (%rsi)
addl (%rdx), %eax
ret
fwprop failed to propagate this because it think cost of address
44(%rdi,%rdx,4) is more expensive than (%rdx), that's correct locally, but
under global view, if it could be propagated into both movl, leaq would be
eliminated, which benifits performance.
The ideal place to handle this issue is TER opt in pass_expand, but currently
TER only handle simple situation ---- single use and block level
48 A pass is made through the function, one block at a time. No cross block
49 information is tracked.
50
51 Variables which only have one use, and whose defining stmt is considered
52 a replaceable expression (see ssa_is_replaceable_p) are tracked to see
whether
53 they can be replaced at their use location.
Should TER be extended?
Another testcase has this issue in more complex cfg
Refer to
https://godbolt.org/z/ofjH9R
