[InstCombine] match usub.sat from umax intrinsic

umax(X, Op1) - Op1 --> usub.sat(X, Op1)

https://alive2.llvm.org/ce/z/HpcGiJ

This happens in 2 or more steps with an icmp-select idiom
instead of an intrinsic. This is another step towards
canonicalization of the min/max intrinsics. See:
D98152

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 6c93969..2c43158 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2168,6 +2168,13 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
     return replaceInstUsesWith(
         I, Builder.CreateIntrinsic(Intrinsic::umin, {I.getType()}, {Op0, Y}));
 
+  // umax(X, Op1) - Op1 --> usub.sat(X, Op1)
+  // TODO: The one-use restriction is not strictly necessary, but it may
+  //       require improving other pattern matching and/or codegen.
+  if (match(Op0, m_OneUse(m_UMax(m_Value(X), m_Specific(Op1)))))
+    return replaceInstUsesWith(
+        I, Builder.CreateIntrinsic(Intrinsic::usub_sat, {Ty}, {X, Op1}));
+
   // C - ctpop(X) => ctpop(~X) if C is bitwidth
   if (match(Op0, m_SpecificInt(Ty->getScalarSizeInBits())) &&
       match(Op1, m_OneUse(m_Intrinsic<Intrinsic::ctpop>(m_Value(X)))))

diff --git a/llvm/test/Transforms/InstCombine/minmax-intrinsics.ll b/llvm/test/Transforms/InstCombine/minmax-intrinsics.ll
index 573ed8e..6c55c23 100644 (file)
--- a/llvm/test/Transforms/InstCombine/minmax-intrinsics.ll
+++ b/llvm/test/Transforms/InstCombine/minmax-intrinsics.ll
@@ -2132,9 +2132,8 @@ define <3 x i8> @umax_vector_splat_undef(<3 x i8> %x) {
 
 define <3 x i8> @umax_sub_vec(<3 x i8> %x, <3 x i8> %y) {
 ; CHECK-LABEL: @umax_sub_vec(
-; CHECK-NEXT:    [[U:%.*]] = call <3 x i8> @llvm.umax.v3i8(<3 x i8> [[X:%.*]], <3 x i8> [[Y:%.*]])
-; CHECK-NEXT:    [[R:%.*]] = sub <3 x i8> [[U]], [[Y]]
-; CHECK-NEXT:    ret <3 x i8> [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call <3 x i8> @llvm.usub.sat.v3i8(<3 x i8> [[X:%.*]], <3 x i8> [[Y:%.*]])
+; CHECK-NEXT:    ret <3 x i8> [[TMP1]]
 ;
   %u = call <3 x i8> @llvm.umax.v3i8(<3 x i8> %x, <3 x i8> %y)
   %r = sub <3 x i8> %u, %y