[InstCombine] reduce more unsigned saturated add with 'not' op

We want to use the sum in the icmp to allow matching with
m_UAddWithOverflow and eliminate the 'not'. This is discussed
in D51929 and is another step towards solving PR14613:
https://bugs.llvm.org/show_bug.cgi?id=14613

  Name: not op
  %notx = xor i32 %x, -1
  %a = add i32 %x, %y
  %c = icmp ult i32 %notx, %y
  %r = select i1 %c, i32 -1, i32 %a
  =>
  %a = add i32 %x, %y
  %c2 = icmp ult i32 %a, %y
  %r = select i1 %c2, i32 -1, i32 %a

  Name: not op ugt
  %notx = xor i32 %x, -1
  %a = add i32 %x, %y
  %c = icmp ugt i32 %y, %notx
  %r = select i1 %c, i32 -1, i32 %a
  =>
  %a = add i32 %x, %y
  %c2 = icmp ult i32 %a, %y
  %r = select i1 %c2, i32 -1, i32 %a

https://rise4fun.com/Alive/niom

(The matching here is still incomplete.)

llvm-svn: 354224

1 files changed, 13 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 8b5dd7923c4..3ee418feb0e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -677,13 +677,23 @@ static Value *canonicalizeSaturatedSubtract(const ICmpInst *ICI,
 
 static Value *canonicalizeSaturatedAdd(ICmpInst *Cmp, Value *TVal, Value *FVal,
                                        InstCombiner::BuilderTy &Builder) {
-  if (!Cmp->hasOneUse() || Cmp->getPredicate() != ICmpInst::ICMP_ULT)
+  if (!Cmp->hasOneUse())
     return nullptr;
 
-  // Match unsigned saturated add of 2 variables with an unnecessary 'not'.
-  // TODO: There are more variations of this pattern.
+  // Canonicalize to 'ULT' to simplify matching below.
   Value *Cmp0 = Cmp->getOperand(0);
   Value *Cmp1 = Cmp->getOperand(1);
+  ICmpInst::Predicate Pred = Cmp->getPredicate();
+  if (Pred == ICmpInst::ICMP_UGT) {
+    Pred = ICmpInst::ICMP_ULT;
+    std::swap(Cmp0, Cmp1);
+  }
+
+  if (Pred != ICmpInst::ICMP_ULT)
+    return nullptr;
+
+  // Match unsigned saturated add of 2 variables with an unnecessary 'not'.
+  // TODO: There are more variations of this pattern.
   Value *X, *Y;
   if (match(TVal, m_AllOnes()) && match(Cmp0, m_Not(m_Value(X))) &&
       match(FVal, m_c_Add(m_Specific(X), m_Value(Y))) && Y == Cmp1) {