[PATCH] [IndVarSimplify] Widen signed loop compare instructions to enable additional optimizations.

[Chad Rosier]

Hi atrick, bkramer, chandlerc, sunfish, compnerd,

This change removes a trunc from in-between an induction variable and the associated compare.  This allows other optimizations (i.e., instcombine, LSR) to take effect. A sext may be added to the compare's other operand, but this can often be hoisted outside of the loop.

For example,
int *ptr;
int e, idx;
int foo() {
  int i;
  idx = -1;
  for (i = 0; i <= e; i++)
    if (!ptr[i]) {
      idx = i;
      break;
    };
  return idx;
}

Before this change, on AArch64 we generate the following loop:

.LBB0_2:                                // %for.body
        ldr     w11, [x10, x0, lsl #2]
        cbz     w11, .LBB0_5
        add     x0, x0, #1           // ++i / pre-increment
        sub     w11, w0, #1        // rematerialize i
        cmp      w11, w9           // compare i
        b.lt    .LBB0_2

After:

.LBB0_2:                                // %for.body
        ldr      w11, [x10]             // remove shift as we can now increment base by 4
        add     x0, x0, #1             // i++ / post-increment i
        cbz     w11, .LBB0_5
        add     x10, x10, #4         // base += 4 
        cmp      x0, x9
        b.lt    .LBB0_2

With a little more work we should be able to generate a post-increment load (my next patch) to generate the following loop:

.LBB0_2:                                // %for.body
        ldr      w11, [x10], 4
        add     x0, x0, #1
        cbz     w11, .LBB0_5
        cmp      x0, x9
        b.lt    .LBB0_2

This change in isolation has a minimal effect on performance (i.e., nothing outside of noise). However, it enables better use of post-increment loads/stores, which is why I deemed it important.  Please have a look.

 Chad

http://reviews.llvm.org/D5333

Files:
  lib/Transforms/Scalar/IndVarSimplify.cpp
  test/Transforms/IndVarSimplify/2011-09-27-hoistsext.ll
  test/Transforms/IndVarSimplify/elim-extend.ll
  test/Transforms/IndVarSimplify/no-iv-rewrite.ll
  test/Transforms/IndVarSimplify/verify-scev.ll
  test/Transforms/IndVarSimplify/widen-loop-comp.ll
  test/Transforms/LoopSimplify/merge-exits.ll
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