[PATCH] D146347: [InstCombine] Improve transforms for `(mul X, Y)` -> `(shl X, log2(Y)`

[Noah Goldstein via Phabricator via llvm-commits]

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Using the more robust log2 search allows us to fold more cases (same
logic as exists for idiv/irem).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D146347

Files:
  llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
  llvm/test/Transforms/InstCombine/mul-pow2.ll


Index: llvm/test/Transforms/InstCombine/mul-pow2.ll
===================================================================
--- llvm/test/Transforms/InstCombine/mul-pow2.ll
+++ llvm/test/Transforms/InstCombine/mul-pow2.ll
@@ -3,9 +3,9 @@
 
 define i8 @mul_x_selectp2(i8 %x, i1 %c) {
 ; CHECK-LABEL: @mul_x_selectp2(
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[C:%.*]], i8 2, i8 4
-; CHECK-NEXT:    [[R:%.*]] = mul i8 [[S]], [[X:%.*]]
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[C:%.*]], i8 1, i8 2
+; CHECK-NEXT:    [[R1:%.*]] = shl i8 [[X:%.*]], [[TMP1]]
+; CHECK-NEXT:    ret i8 [[R1]]
 ;
   %s = select i1 %c, i8 2, i8 4
   %r = mul i8 %x, %s
@@ -14,10 +14,9 @@
 
 define i8 @mul_x_selectp2_non_const(i8 %x, i1 %c, i8 %yy) {
 ; CHECK-LABEL: @mul_x_selectp2_non_const(
-; CHECK-NEXT:    [[Y:%.*]] = shl nuw i8 1, [[YY:%.*]]
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[C:%.*]], i8 2, i8 [[Y]]
-; CHECK-NEXT:    [[R:%.*]] = mul i8 [[S]], [[X:%.*]]
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[C:%.*]], i8 1, i8 [[YY:%.*]]
+; CHECK-NEXT:    [[R1:%.*]] = shl i8 [[X:%.*]], [[TMP1]]
+; CHECK-NEXT:    ret i8 [[R1]]
 ;
   %y = shl i8 1, %yy
   %s = select i1 %c, i8 2, i8 %y
@@ -28,9 +27,9 @@
 define i8 @mul_selectp2_x(i8 %xx, i1 %c) {
 ; CHECK-LABEL: @mul_selectp2_x(
 ; CHECK-NEXT:    [[X:%.*]] = mul i8 [[XX:%.*]], [[XX]]
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[C:%.*]], i8 8, i8 1
-; CHECK-NEXT:    [[R:%.*]] = mul i8 [[S]], [[X]]
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[C:%.*]], i8 3, i8 0
+; CHECK-NEXT:    [[R1:%.*]] = shl i8 [[X]], [[TMP1]]
+; CHECK-NEXT:    ret i8 [[R1]]
 ;
   %x = mul i8 %xx, %xx
   %s = select i1 %c, i8 8, i8 1
@@ -52,9 +51,9 @@
 define <2 x i8> @mul_selectp2_x_vec(<2 x i8> %xx, i1 %c) {
 ; CHECK-LABEL: @mul_selectp2_x_vec(
 ; CHECK-NEXT:    [[X:%.*]] = mul <2 x i8> [[XX:%.*]], [[XX]]
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[C:%.*]], <2 x i8> <i8 8, i8 16>, <2 x i8> <i8 4, i8 1>
-; CHECK-NEXT:    [[R:%.*]] = mul <2 x i8> [[S]], [[X]]
-; CHECK-NEXT:    ret <2 x i8> [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[C:%.*]], <2 x i8> <i8 3, i8 4>, <2 x i8> <i8 2, i8 0>
+; CHECK-NEXT:    [[R1:%.*]] = shl <2 x i8> [[X]], [[TMP1]]
+; CHECK-NEXT:    ret <2 x i8> [[R1]]
 ;
   %x = mul <2 x i8> %xx, %xx
   %s = select i1 %c, <2 x i8> <i8 8, i8 16>, <2 x i8> <i8 4, i8 1>
Index: llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -185,6 +185,9 @@
   return nullptr;
 }
 
+static Value *takeLog2(IRBuilderBase &Builder, Value *Op, unsigned Depth,
+                       bool DoFold);
+
 Instruction *InstCombinerImpl::visitMul(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   if (Value *V =
@@ -471,6 +474,20 @@
   if (Instruction *Ext = narrowMathIfNoOverflow(I))
     return Ext;
 
+  // (mul Op0 Op1):
+  //    if Log2(Op0) folds away ->
+  //        (shl Op1, Log2(Op0))
+  //    if Log2(Op1) folds away ->
+  //        (shl Op0, Log2(Op1))
+  if (takeLog2(Builder, Op0, /*Depth*/ 0, /*DoFold*/ false)) {
+    Value *Res = takeLog2(Builder, Op0, /*Depth*/ 0, /*DoFold*/ true);
+    return replaceInstUsesWith(I, Builder.CreateShl(Op1, Res, I.getName()));
+  }
+  if (takeLog2(Builder, Op1, /*Depth*/ 0, /*DoFold*/ false)) {
+    Value *Res = takeLog2(Builder, Op1, /*Depth*/ 0, /*DoFold*/ true);
+    return replaceInstUsesWith(I, Builder.CreateShl(Op0, Res, I.getName()));
+  }
+
   bool Changed = false;
   if (!HasNSW && willNotOverflowSignedMul(Op0, Op1, I)) {
     Changed = true;


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