[InstCombine] Combine neg of shl of sub (PR44529)

Fixes https://bugs.llvm.org/show_bug.cgi?id=44529. We already have
a combine to sink a negation through a left-shift, but it currently
only works if the shift operand is negatable without creating any
instructions. This patch introduces freelyNegateValue() as a more
powerful extension of dyn_castNegVal(), which allows negating a
value as long as this doesn't end up increasing instruction count.
Specifically, this patch adds support for negating A-B to B-A.

This mechanism could in the future be extended to handle general
negation chains that a) start at a proper 0-X negation and b) only
require one operand to be freely negatable. This would end up as a
weaker form of D68408 aimed at the most obviously profitable subset
that eliminates a negation entirely.

Differential Revision: https://reviews.llvm.org/D72978

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1985,7 +1985,7 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
 
     // 0 - (X << Y)  -> (-X << Y)   when X is freely negatable.
     if (match(Op1, m_Shl(m_Value(X), m_Value(Y))) && match(Op0, m_Zero()))
-      if (Value *XNeg = dyn_castNegVal(X))
+      if (Value *XNeg = freelyNegateValue(X))
         return BinaryOperator::CreateShl(XNeg, Y);
 
     // Subtracting -1/0 is the same as adding 1/0:

llvm/lib/Transforms/InstCombine/InstCombineInternal.h

@@ -474,6 +474,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner
   bool shouldChangeType(unsigned FromBitWidth, unsigned ToBitWidth) const;
   bool shouldChangeType(Type *From, Type *To) const;
   Value *dyn_castNegVal(Value *V) const;
+  Value *freelyNegateValue(Value *V);
   Type *FindElementAtOffset(PointerType *PtrTy, int64_t Offset,
                             SmallVectorImpl<Value *> &NewIndices);
 

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -856,6 +856,23 @@ Value *InstCombiner::dyn_castNegVal(Value *V) const {
   return nullptr;
 }
 
+/// Get negated V (that is 0-V) without increasing instruction count,
+/// assuming that the original V will become unused.
+Value *InstCombiner::freelyNegateValue(Value *V) {
+  if (Value *NegV = dyn_castNegVal(V))
+    return NegV;
+
+  if (!V->hasOneUse())
+    return nullptr;
+
+  Value *A, *B;
+  // 0-(A-B)  =>  B-A
+  if (match(V, m_Sub(m_Value(A), m_Value(B))))
+    return Builder.CreateSub(B, A);
+
+  return nullptr;
+}
+
 static Value *foldOperationIntoSelectOperand(Instruction &I, Value *SO,
                                              InstCombiner::BuilderTy &Builder) {
   if (auto *Cast = dyn_cast<CastInst>(&I))

llvm/test/Transforms/InstCombine/sub.ll

@@ -543,9 +543,8 @@ define i32 @test26(i32 %x) {
 
 define i64 @test_neg_shl_sub(i64 %a, i64 %b) {
 ; CHECK-LABEL: @test_neg_shl_sub(
-; CHECK-NEXT:    [[SUB:%.*]] = sub i64 [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[MUL:%.*]] = shl i64 [[SUB]], 2
-; CHECK-NEXT:    [[NEG:%.*]] = sub i64 0, [[MUL]]
+; CHECK-NEXT:    [[TMP1:%.*]] = sub i64 [[B:%.*]], [[A:%.*]]
+; CHECK-NEXT:    [[NEG:%.*]] = shl i64 [[TMP1]], 2
 ; CHECK-NEXT:    ret i64 [[NEG]]
 ;
   %sub = sub i64 %a, %b