[llvm-bugs] [Bug 32971] New: [ppc] Highly predictable ISEL should be changed to cmp/br

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=32971

            Bug ID: 32971
           Summary: [ppc] Highly predictable ISEL should be changed to
                    cmp/br
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: PowerPC
          Assignee: unassignedbugs at nondot.org
          Reporter: carrot at google.com
                CC: llvm-bugs at lists.llvm.org

On power, use ISEL or cmp/br is a general hard problem. But if we can point out
the branch is highly predictable (either through runtime profiling or static
estimate), we should use cmp/br.

Following is a test case simplified from real world code.

#include <vector>

typedef std::vector<int> MyVec;
int foo(int iters, MyVec* vec) {
  MyVec::const_iterator it = vec->begin();

  for (int i = 0; i < iters; ++i, ++it) {
    if (it == vec->end()) {
      it = vec->begin();
    }   
    if (*it >= iters)
      return 1;
  }
  return 0;
}

The iterator "it" is incremented in each iteration, and it is compared to the
same value vec->end(), so we can expect the condition is false for most times,
and then it is true, and then false again ...
So this branch is highly predictable. In our code, cmp/br is 2x faster than
isel.

LLVM generates following code, which uses ISEL

# BB#0:                                 # %entry
        cmpwi    3, 1
        blt      0, .LBB0_4
# BB#1:                                 # %for.body.lr.ph
        addi 6, 3, -1
        ld 5, 0(4)
        ld 4, 8(4)
        clrldi   6, 6, 32
        addi 6, 6, 1
        mr 7, 5
        mtctr 6
        li 6, 0
        .p2align        5
.LBB0_2:                                # %for.body
                                        # =>This Inner Loop Header: Depth=1
        cmpld    4, 7
        isel 7, 5, 7, 2
        lwz 8, 0(7)
        cmpw     8, 3
        bge 0, .LBB0_5
# BB#3:                                 # %for.inc
                                        #   in Loop: Header=BB0_2 Depth=1
        addi 6, 6, 1
        addi 7, 7, 4
        bdnz .LBB0_2
.LBB0_4:
        li 3, 0
        clrldi   3, 3, 32
        blr
.LBB0_5:
        li 3, 1
        clrldi   3, 3, 32
        blr

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