https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105496
Bug ID: 105496
Summary: Comparison optimizations result in unnecessary cmp
instructions
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: redbeard0531 at gmail dot com
Target Milestone: ---
https://godbolt.org/z/1zdYsaqEj
Consider these equivalent functions:
int test1(int x) {
if (x <= 10)
return 123;
if (x == 11)
return 456;
return 789;
}
int test2(int x) {
if (x < 11)
return 123;
if (x == 11)
return 456;
return 789;
}
In test2 it is very clear that you can do a single cmp of x with 11 then use
different flag bits to choose your case. In test1 it is less clear, but because
x<=10 and x<11 are equivalent, you could always transform one to the other.
Clang seems to do this correctly and transforms test1 into test2 and only emits
a single cmp instruction in each. For some reason, not only does gcc miss this
optimization, it seems to go the other direction and transform test2 into
test1, emitting 2 cmp instructions for both!
test1(int):
mov eax, 123
cmp edi, 10
jle .L1
cmp edi, 11
mov eax, 456
mov edx, 789
cmovne eax, edx
.L1:
ret
test2(int):
mov eax, 123
cmp edi, 10
jle .L6
cmp edi, 11
mov eax, 456
mov edx, 789
cmovne eax, edx
.L6:
ret
Observed with at least -O2 and -O3. I initially observed this for code where
each if generated an actual branch rather than a cmov, but when I reduced the
example, the cmov was generated.
I'm not sure if this should be a middle-end or target specific optimization,
since ideally it would be smart on all targets that use comparison flags, even
if there are some targets that don't. Is there ever a down side to trying to
make two adjacent comparisons compare the same number?
