[PATCH] D55009: [GVN] Don't perform scalar PRE on GEPs

[John Brawn via Phabricator via llvm-commits]

john.brawn added a comment.

The idea behind allowing Scalar PRE when the addressing mode isn't legal is for some thing like (adjusted from pre-gep-load.ll)

  define void @foo(i32 %stat, i32 %i, double* %p, double* %q) {
  entry:
    switch i32 %stat, label %sw.default [
      i32 0, label %sw.bb
      i32 1, label %sw.bb
      i32 2, label %sw.bb2
    ]
  
  sw.bb:                                            ; preds = %entry, %entry
    %arrayidx1 = getelementptr inbounds double, double* %p, i64 1234567
    store double 1.0, double* %arrayidx1, align 8
    br i1 undef, label %if.then, label %if.end
  
  if.then:                                          ; preds = %sw.bb
    br label %return
  
  if.end:                                           ; preds = %sw.bb
    br label %sw.bb2
  
  sw.bb2:                                           ; preds = %if.end, %entry
    %arrayidx5 = getelementptr inbounds double, double* %p, i64 1234567
    store double 0.0, double* %arrayidx5, align 8
    br label %return
  
  sw.default:                                       ; preds = %entry
    br label %return
  
  return:                                           ; preds = %sw.default, %sw.bb2, %if.then
    ret void
  }

The offset in the gep is too big to use as an immediate offset in AArch64, so we'd like to not materialise the same constant twice to use as the offset, but doing PRE on the GEP means we get

  	mov	w8, #46136
  	movk	w8, #150, lsl #16
  	add	x8, x2, x8
  .LBB0_4:                                // %sw.bb2
  	str	xzr, [x8]

but we really want

  	mov	w8, #46136
  	movk	w8, #150, lsl #16
  .LBB0_4:                                // %sw.bb2
  	str	xzr, [x2, x8]

i.e. we'd like to PRE the offset generation but not the add. So I think that side of things is better handled by some kind of MachinePRE, and for GVN it's fine to just refuse to do scalar PRE on GEPs (i.e. what this patch does).


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  https://reviews.llvm.org/D55009/new/

https://reviews.llvm.org/D55009