[Bug tree-optimization/81346] New: Missed constant propagation into comparison

[gergo.barany at inria dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81346

            Bug ID: 81346
           Summary: Missed constant propagation into comparison
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gergo.barany at inria dot fr
  Target Milestone: ---

Created attachment 41694
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41694&action=edit
Input C file for triggering the issue

The attached C file contains the following function:

int fn1(int p1) {
  int b = (p1 / 12 == 6);
  return b;
}

As these are integers, the expression (p1 / 12 == 6) can be optimized to a
subtraction and an unsigned compare. GCC can do this (here for ARM):

fn1:
        sub     r0, r0, #72
        cmp     r0, #11
        movhi   r0, #0
        movls   r0, #1
        bx      lr

The attached file also contains the following function:

int fn2(int p1) {
  int a = 6;
  int b = (p1 / 12 == a);
  return b;
}

This is equivalent to the above code; the value of a can only ever be 6.
Consequently, the output machine code should be equivalent. However, GCC does
not recognize the above pattern and generates more complex code:

fn2: 
        movw    r3, #43691
        movt    r3, 10922
        smull   r2, r3, r3, r0
        asr     r0, r0, #31
        rsb     r0, r0, r3, asr #1
        sub     r0, r0, #6
        clz     r0, r0
        lsr     r0, r0, #5
        bx      lr

I believe this is a target-independent optimization issue because x86-64 and
PowerPC behave analogously, for example (x86-64):

fn1:
        subl    $72, %edi
        xorl    %eax, %eax
        cmpl    $11, %edi
        setbe   %al
        ret

fn2:
        movl    %edi, %eax
        movl    $715827883, %edx
        sarl    $31, %edi
        imull   %edx
        xorl    %eax, %eax
        sarl    %edx
        subl    %edi, %edx
        cmpl    $6, %edx
        sete    %al
        ret

Version:
gcc version 8.0.0 20170706 (experimental) (GCC)
Configured with: --target=armv7a-eabihf --with-arch=armv7-a
--with-fpu=vfpv3-d16 --with-float-abi=hard --with-float=hard
or
with: --target=x86_64-pc-linux-gnu
or
with: --target=ppc-eabi