[PATCH] D123453: [InstCombine] Fold mul nuw+lshr to a single multiplication when the latter is a factor

[chenglin.bi via Phabricator via llvm-commits]

bcl5980 updated this revision to Diff 423835.
bcl5980 added a comment.

update comments and rebase


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D123453/new/

https://reviews.llvm.org/D123453

Files:
  llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
  llvm/test/Transforms/InstCombine/shift-logic.ll


Index: llvm/test/Transforms/InstCombine/shift-logic.ll
===================================================================
--- llvm/test/Transforms/InstCombine/shift-logic.ll
+++ llvm/test/Transforms/InstCombine/shift-logic.ll
@@ -259,9 +259,8 @@
 
 define i64 @lshr_mul(i64 %0) {
 ; CHECK-LABEL: @lshr_mul(
-; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw i64 [[TMP0:%.*]], 52
-; CHECK-NEXT:    [[TMP3:%.*]] = lshr exact i64 [[TMP2]], 2
-; CHECK-NEXT:    ret i64 [[TMP3]]
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw i64 [[TMP0:%.*]], 13
+; CHECK-NEXT:    ret i64 [[TMP2]]
 ;
   %2 = mul nuw i64 %0, 52
   %3 = lshr i64 %2, 2
@@ -270,9 +269,8 @@
 
 define i64 @lshr_mul_nuw_nsw(i64 %0) {
 ; CHECK-LABEL: @lshr_mul_nuw_nsw(
-; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw nsw i64 [[TMP0:%.*]], 52
-; CHECK-NEXT:    [[TMP3:%.*]] = lshr exact i64 [[TMP2]], 2
-; CHECK-NEXT:    ret i64 [[TMP3]]
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw i64 [[TMP0:%.*]], 13
+; CHECK-NEXT:    ret i64 [[TMP2]]
 ;
   %2 = mul nuw nsw i64 %0, 52
   %3 = lshr i64 %2, 2
@@ -281,9 +279,8 @@
 
 define <4 x i32> @lshr_mul_vector(<4 x i32> %0) {
 ; CHECK-LABEL: @lshr_mul_vector(
-; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw <4 x i32> [[TMP0:%.*]], <i32 52, i32 52, i32 52, i32 52>
-; CHECK-NEXT:    [[TMP3:%.*]] = lshr exact <4 x i32> [[TMP2]], <i32 2, i32 2, i32 2, i32 2>
-; CHECK-NEXT:    ret <4 x i32> [[TMP3]]
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw <4 x i32> [[TMP0:%.*]], <i32 13, i32 13, i32 13, i32 13>
+; CHECK-NEXT:    ret <4 x i32> [[TMP2]]
 ;
   %2 = mul nuw <4 x i32> %0, <i32 52, i32 52, i32 52, i32 52>
   %3 = lshr <4 x i32> %2, <i32 2, i32 2, i32 2, i32 2>
@@ -324,3 +321,14 @@
   %3 = lshr i64 %2, 2
   ret i64 %3
 }
+
+define i64 @lshr_mul_negative_nsw(i64 %0) {
+; CHECK-LABEL: @lshr_mul_negative_nsw(
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nsw i64 [[TMP0:%.*]], 52
+; CHECK-NEXT:    [[TMP3:%.*]] = lshr exact i64 [[TMP2]], 2
+; CHECK-NEXT:    ret i64 [[TMP3]]
+;
+  %2 = mul nsw i64 %0, 52
+  %3 = lshr i64 %2, 2
+  ret i64 %3
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -1164,15 +1164,24 @@
       }
     }
 
-    // Look for a "splat" mul pattern - it replicates bits across each half of
-    // a value, so a right shift is just a mask of the low bits:
-    // lshr i[2N] (mul nuw X, (2^N)+1), N --> and iN X, (2^N)-1
-    // TODO: Generalize to allow more than just half-width shifts?
     const APInt *MulC;
-    if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&
-        BitWidth > 2 && ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
-        MulC->logBase2() == ShAmtC)
-      return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
+    if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC)))) {
+      // Look for a "splat" mul pattern - it replicates bits across each half of
+      // a value, so a right shift is just a mask of the low bits:
+      // lshr i[2N] (mul nuw X, (2^N)+1), N --> and iN X, (2^N)-1
+      // TODO: Generalize to allow more than just half-width shifts?
+      if (BitWidth > 2 && ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
+          MulC->logBase2() == ShAmtC)
+        return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
+
+      if (Op0->hasOneUse()) {
+        APInt NewMulC = MulC->lshr(ShAmtC);
+        // if c is divisible by (1 << ShAmtC):
+        // lshr (mul nuw x, MulC), ShAmtC -> mul nuw x, (MulC >> ShAmtC)
+        if (MulC->eq(NewMulC.shl(ShAmtC)))
+          return BinaryOperator::CreateNUWMul(X, ConstantInt::get(Ty, NewMulC));
+      }
+    }
 
     // Try to narrow a bswap:
     // (bswap (zext X)) >> C --> zext (bswap X >> C')


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