[Bug tree-optimization/116546] New: Missed optimization of redundant comparison

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116546

            Bug ID: 116546
           Summary: Missed optimization of redundant comparison
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bergner at gcc dot gnu.org
  Target Milestone: ---

In the following test case, the "n & 4" test is redundant and should be
eliminated, since the "n &= 7;" statement limits n's potential values to [0,7].
 The "n >= 4" test further narrows its potential values to [4,7], making the "n
& 4" test always true, allowing the elimination of the code path calling bar(). 

bergner@ltcden2-lp1:~$ cat test.c 
extern long foo (void);
extern long bar (void);

long
test (long n)
{
  n &= 7;
  if (n >= 4) {
    if (n & 4)
      return foo ();
    else
      return bar ();
  }
  return 0;
}

Current gcc (-O2) does optimize this on powerpc64le-linux and removes the bar()
code path. However, if we change the "n >= 4" test to either "n > 4" or "n ==
4", then we fail to eliminate the code path to bar(), even though the "n & 4"
test remains redundant.

Using -O2 -mcpu=power10 (-mcpu=power10 allows tail calls making the resulting
asm easier to read, but it isn't required) produces for the "n >= 4" test:

test:
        andi. 3,3,0x4
        bne 0,.L4
        li 3,0
        blr
        .p2align 4,,15
.L4:
        b foo@notoc

Where as the "n > 4" test results in:

test:
        rldicl 2,3,0,61
        cmpdi 0,2,4
        ble 0,.L2
        andi. 3,3,0x4
        beq 0,.L3
        b foo@notoc
        .p2align 4,,15
.L2:
        li 3,0
        blr
        .p2align 4,,15
.L3:
        b bar@notoc

Interestingly to me, the optimized code keeps the inner redundant "n & 4" test
(andi. 3,3,0x4) and removes the outer tests.  I didn't expect that.