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

[gergo.barany at inria dot fr]

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

--- Comment #17 from Gergö Barany <gergo.barany at inria dot fr> ---
Thanks for fixing this. I did notice a small thing that might be considered a
tiny regression due to the fix.

If the divisor is a small power of 2, as in the following example:

int fn1(char p1) {
  long a;
  char b;
  int c = a = 4;
  b = !(p1 / a);
  if (b)
    c = 0;
  return c;
}

the division used to be replaced by a shift that updated the condition code
register (again, on ARM; r250337):

    lsrs        r3, r0, #2
    movne       r0, #4
    moveq       r0, #0
    bx  lr

whereas after the fix (tested on r250342) the new folding rule takes precedence
and generates one instruction more:

    add r0, r0, #3
    cmp r0, #6
    movhi       r0, #4
    movls       r0, #0
    bx  lr

I guess the rule could be updated to only apply if the divisor is not a small
power of 2, or folding a division by a power of 2 into a shift could be
prioritized.

Sorry about only pointing this out two months later! Also, let me stress that I
do not have code that depends on this transformation. This came out of research
I'm doing on missed optimization, and this was one example I found interesting.