[PATCH] D158389: [InstCombine] Extend sub(sext(add(X, Y)), sext(add(X, Z))) -> sub(Y, Z) fold to adds that mutually wrap

[Dave Green via Phabricator via llvm-commits]

dmgreen created this revision.
dmgreen added reviewers: nikic, spatel, goldstein.w.n.
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This combine, added in D157598 <https://reviews.llvm.org/D157598>, folds the sub of two extended adds if the adds are no wrap, as we know the extends are simple. This isn't just valid for no-wrap though - it is also valid if the two adds wrap at the same time. i.e given the following code:

  ;  %a = srem i32 %X, 3
  ;  switch i32 %a, label %.loopexit477 [
  ;    i32 1, label %preheader
  ;    i32 2, label %.loopexit477.loopexit667
  ;  ]
  ;
  ;preheader:
  ;  %b = add i32 %X, 1
  ;  %c = sext i32 %b to i64
  ;  %d = add i32 %X, 2
  ;  %e = sext i32 %d to i64
  ;  %f = sub nsw i64 %e, %c

The sub is provably '1' because due to knowing from the dominating condition that X % 3 == 1, and that for i32 the two adds will either both not wrap (for most values of X), or both wrap if X==2147483647. https://alive2.llvm.org/ce/z/JvnY-o

So this patch adds a isKnownMutuallyWrapping function to detect when two adds will wrap at the same time, and from the mod case detects if the two adds will wrap using `(C3 - (Max % C4) + C1) / C4 == (C3 - (Max % C4) + C2) / C4`, where C4 is the mod denom and C3 is the remainder.
For Signed: https://alive2.llvm.org/ce/z/N93Njy
And Unsigned: https://alive2.llvm.org/ce/z/A9gzbK (note this is more generally true for the conditions here, but this patch only tries to prove it for the mutually wrapping case).


https://reviews.llvm.org/D158389

Files:
  llvm/include/llvm/Analysis/ValueTracking.h
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/test/Transforms/InstCombine/sub-ext-add.ll

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