[InstCombine] fold negate disguised as select+mul

  Name: negate if true
  %sel = select i1 %cond, i32 -1, i32 1
  %r = mul i32 %sel, %x
  =>
  %m = sub i32 0, %x
  %r = select i1 %cond, i32 %m, i32 %x

  Name: negate if false
  %sel = select i1 %cond, i32 1, i32 -1
  %r = mul i32 %sel, %x
  =>
  %m = sub i32 0, %x
  %r = select i1 %cond, i32 %x, i32 %m

https://rise4fun.com/Alive/Nlh

llvm-svn: 373230

2 files changed, 31 insertions, 8 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 7f152aac52d..a301d9eef60 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -213,6 +213,25 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
   if (Instruction *FoldedMul = foldBinOpIntoSelectOrPhi(I))
     return FoldedMul;
 
+  // TODO: This is a specific form of a much more general pattern.
+  //       We could detect a select with any binop identity constant, or we
+  //       could use SimplifyBinOp to see if either arm of the select reduces.
+  //       But that needs to be done carefully and/or while removing potential
+  //       reverse canonicalizations as in InstCombiner::foldSelectIntoOp().
+  // mul (select Cond, 1, -1), Op1 --> select Cond, Op1, -Op1
+  // mul (select Cond, -1, 1), Op1 --> select Cond, -Op1, Op1
+  // mul Op0, (select Cond, 1, -1) --> select Cond, Op0, -Op0
+  // mul Op0, (select Cond, -1, 1) --> select Cond, -Op0, Op0
+  Value *Cond;
+  if (match(Op0, m_OneUse(m_Select(m_Value(Cond), m_One(), m_AllOnes()))))
+    return SelectInst::Create(Cond, Op1, Builder.CreateNeg(Op1));
+  if (match(Op0, m_OneUse(m_Select(m_Value(Cond), m_AllOnes(), m_One()))))
+    return SelectInst::Create(Cond, Builder.CreateNeg(Op1), Op1);
+  if (match(Op1, m_OneUse(m_Select(m_Value(Cond), m_One(), m_AllOnes()))))
+    return SelectInst::Create(Cond, Op0, Builder.CreateNeg(Op0));
+  if (match(Op1, m_OneUse(m_Select(m_Value(Cond), m_AllOnes(), m_One()))))
+    return SelectInst::Create(Cond, Builder.CreateNeg(Op0), Op0);
+
   // Simplify mul instructions with a constant RHS.
   if (isa<Constant>(Op1)) {
     // Canonicalize (X+C1)*CI -> X*CI+C1*CI.
diff --git a/llvm/test/Transforms/InstCombine/mul.ll b/llvm/test/Transforms/InstCombine/mul.ll
index 37f05d222c9..f106026762b 100644
--- a/llvm/test/Transforms/InstCombine/mul.ll
+++ b/llvm/test/Transforms/InstCombine/mul.ll
@@ -520,8 +520,8 @@ define i64 @test_mul_canonicalize_neg_is_not_undone(i64 %L1) {
 
 define i32 @negate_if_true(i32 %x, i1 %cond) {
 ; CHECK-LABEL: @negate_if_true(
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 -1, i32 1
-; CHECK-NEXT:    [[R:%.*]] = mul i32 [[SEL]], [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = sub i32 0, [[X:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[COND:%.*]], i32 [[TMP1]], i32 [[X]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %sel = select i1 %cond, i32 -1, i32 1
@@ -531,8 +531,8 @@ define i32 @negate_if_true(i32 %x, i1 %cond) {
 
 define i32 @negate_if_false(i32 %x, i1 %cond) {
 ; CHECK-LABEL: @negate_if_false(
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 1, i32 -1
-; CHECK-NEXT:    [[R:%.*]] = mul i32 [[SEL]], [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = sub i32 0, [[X:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[COND:%.*]], i32 [[X]], i32 [[TMP1]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %sel = select i1 %cond, i32 1, i32 -1
@@ -543,8 +543,8 @@ define i32 @negate_if_false(i32 %x, i1 %cond) {
 define <2 x i8> @negate_if_true_commute(<2 x i8> %px, i1 %cond) {
 ; CHECK-LABEL: @negate_if_true_commute(
 ; CHECK-NEXT:    [[X:%.*]] = sdiv <2 x i8> <i8 42, i8 42>, [[PX:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], <2 x i8> <i8 -1, i8 -1>, <2 x i8> <i8 1, i8 1>
-; CHECK-NEXT:    [[R:%.*]] = mul <2 x i8> [[X]], [[SEL]]
+; CHECK-NEXT:    [[TMP1:%.*]] = sub nsw <2 x i8> zeroinitializer, [[X]]
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[COND:%.*]], <2 x i8> [[TMP1]], <2 x i8> [[X]]
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %x = sdiv <2 x i8> <i8 42, i8 42>, %px  ; thwart complexity-based canonicalization
@@ -556,8 +556,8 @@ define <2 x i8> @negate_if_true_commute(<2 x i8> %px, i1 %cond) {
 define <2 x i8> @negate_if_false_commute(<2 x i8> %px, <2 x i1> %cond) {
 ; CHECK-LABEL: @negate_if_false_commute(
 ; CHECK-NEXT:    [[X:%.*]] = sdiv <2 x i8> <i8 42, i8 5>, [[PX:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = select <2 x i1> [[COND:%.*]], <2 x i8> <i8 1, i8 undef>, <2 x i8> <i8 -1, i8 -1>
-; CHECK-NEXT:    [[R:%.*]] = mul <2 x i8> [[X]], [[SEL]]
+; CHECK-NEXT:    [[TMP1:%.*]] = sub <2 x i8> zeroinitializer, [[X]]
+; CHECK-NEXT:    [[R:%.*]] = select <2 x i1> [[COND:%.*]], <2 x i8> [[X]], <2 x i8> [[TMP1]]
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %x = sdiv <2 x i8> <i8 42, i8 5>, %px  ; thwart complexity-based canonicalization
@@ -566,6 +566,8 @@ define <2 x i8> @negate_if_false_commute(<2 x i8> %px, <2 x i1> %cond) {
   ret <2 x i8> %r
 }
 
+; Negative test
+
 define i32 @negate_if_true_extra_use(i32 %x, i1 %cond) {
 ; CHECK-LABEL: @negate_if_true_extra_use(
 ; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 -1, i32 1
@@ -579,6 +581,8 @@ define i32 @negate_if_true_extra_use(i32 %x, i1 %cond) {
   ret i32 %r
 }
 
+; Negative test
+
 define <2 x i8> @negate_if_true_wrong_constant(<2 x i8> %px, i1 %cond) {
 ; CHECK-LABEL: @negate_if_true_wrong_constant(
 ; CHECK-NEXT:    [[X:%.*]] = sdiv <2 x i8> <i8 42, i8 42>, [[PX:%.*]]