[InstCombine] Sink sub into hands of select if one hand becomes zero. Part 2 (PR44426)

This decreases use count of %Op0, makes one hand of select to be 0,
and possibly exposes further folding potential.

Name: sub %Op0, (select %Cond, %Op0, %FalseVal) -> select %Cond, 0, (sub %Op0, %FalseVal)
  %Op0 = %TrueVal
  %o = select i1 %Cond, i8 %Op0, i8 %FalseVal
  %r = sub i8 %Op0, %o
=>
  %n = sub i8 %Op0, %FalseVal
  %r = select i1 %Cond, i8 0, i8 %n

Name: sub %Op0, (select %Cond, %TrueVal, %Op0) -> select %Cond, (sub %Op0, %TrueVal), 0
  %Op0 = %FalseVal
  %o = select i1 %Cond, i8 %TrueVal, i8 %Op0
  %r = sub i8 %Op0, %o
=>
  %n = sub i8 %Op0, %TrueVal
  %r = select i1 %Cond, i8 %n, i8 0

https://rise4fun.com/Alive/aHRt

https://bugs.llvm.org/show_bug.cgi?id=44426

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 05d13aeabe5..c723ba0a683 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1899,6 +1899,33 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
   }
 
   {
+    // If we are subtracting the select with one hand matching the value
+    // we are subtracting from, sink subtraction into hands of select:
+    //   sub %Op0, (select %Cond, %TrueVal, %FalseVal)
+    //     ->
+    //   select %Cond, (sub %Op0, %TrueVal), (sub %Op0, %FalseVal)
+    // This will result in select between new subtraction and 0.
+    Value *Cond, *TrueVal, *FalseVal;
+    if (match(Op1, m_OneUse(m_Select(m_Value(Cond), m_Value(TrueVal),
+                                     m_Value(FalseVal)))) &&
+        (Op0 == TrueVal || Op0 == FalseVal)) {
+      // While it is really tempting to just create two subtractions and let
+      // InstCombine fold one of those to 0, it isn't possible to do so
+      // because of worklist visitation order. So ugly it is.
+      bool SubtractingFromTrueVal = Op0 == TrueVal;
+      Value *NewSub =
+          Builder.CreateSub(Op0, SubtractingFromTrueVal ? FalseVal : TrueVal);
+      Constant *Zero = Constant::getNullValue(I.getType());
+      SelectInst *NewSel =
+          SelectInst::Create(Cond, SubtractingFromTrueVal ? Zero : NewSub,
+                             SubtractingFromTrueVal ? NewSub : Zero);
+      // Preserve prof metadata if any.
+      NewSel->copyMetadata(cast<Instruction>(*Op1));
+      return NewSel;
+    }
+  }
+
+  {
     // If we are subtracting from select with one hand matching the value
     // we are subtracting, sink subtraction into hands of select:
     //   sub (select %Cond, %TrueVal, %FalseVal), %Op1