[PATCH] D36679: [InstCombine] Added support for: trunc(ashr(mul(sext(...), sext(...))) -> ashr(mul(...))

[Amjad Aboud via Phabricator via llvm-commits]

aaboud updated this revision to Diff 111512.
aaboud added a comment.

After committing the other parts, this patch was reduced to handle only the multiply in ComputeNumSignBitsImpl.


https://reviews.llvm.org/D36679

Files:
  lib/Analysis/ValueTracking.cpp
  test/Transforms/InstCombine/trunc.ll


Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -2196,6 +2196,16 @@
     if (Tmp == 1) return 1;  // Early out.
     return std::min(Tmp, Tmp2)-1;
 
+  case Instruction::Mul: {
+    // The output of the Mul can at most twice the valid bits in the inputs.
+    Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
+    if (Tmp == 1) return 1;  // Early out.
+    Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth + 1, Q);
+    if (Tmp2 == 1) return 1;
+    unsigned OutValidBits = (TyBits - Tmp + 1) + (TyBits - Tmp2 + 1);
+    return OutValidBits > TyBits ? 1 : TyBits - OutValidBits + 1;
+  }
+
   case Instruction::PHI: {
     const PHINode *PN = cast<PHINode>(U);
     unsigned NumIncomingValues = PN->getNumIncomingValues();
Index: test/Transforms/InstCombine/trunc.ll
===================================================================
--- test/Transforms/InstCombine/trunc.ll
+++ test/Transforms/InstCombine/trunc.ll
@@ -89,6 +89,21 @@
   ret i32 %D
 }
 
+define i16 @ashr_mul(i8 %X, i8 %Y) {
+; CHECK-LABEL: @ashr_mul(
+; CHECK-NEXT:    [[A:%.*]] = sext i8 %X to i16
+; CHECK-NEXT:    [[B:%.*]] = sext i8 %Y to i16
+; CHECK-NEXT:    [[C:%.*]] = mul nsw i16 [[A]], [[B]]
+; CHECK-NEXT:    [[D:%.*]] = ashr i16 [[C]], 8
+; CHECK-NEXT:    ret i16 [[D]]
+  %A = sext i8 %X to i20
+  %B = sext i8 %Y to i20
+  %C = mul i20 %A, %B
+  %D = ashr i20 %C, 8
+  %E = trunc i20 %D to i16
+  ret i16 %E
+}
+
 define i32 @trunc_ashr(i32 %X) {
 ; CHECK-LABEL: @trunc_ashr(
 ; CHECK-NEXT:    [[B:%.*]] = or i32 [[X:%.*]], -2147483648


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