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From: "redbeard0531 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/105496] New: Comparison optimizations result in
 unnecessary cmp instructions
Date: Thu, 05 May 2022 14:18:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105496

            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 <=3D 10)
        return 123;
    if (x =3D=3D 11)
        return 456;
    return 789;
}

int test2(int x) {
    if (x < 11)
        return 123;
    if (x =3D=3D 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 bec=
ause
x<=3D10 and x<11 are equivalent, you could always transform one to the othe=
r.
Clang seems to do this correctly and transforms test1 into test2 and only e=
mits
a single cmp instruction in each. For some reason, not only does gcc miss t=
his
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 t=
he
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, e=
ven
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?=