[PATCH] D91343: [InstCombine] Optimize away the unnecessary multi-use sign-extend

[Bhramar Vatsa via Phabricator via llvm-commits]

Bhramar.vatsa updated this revision to Diff 305671.
Bhramar.vatsa added a comment.

Modified the comment as suggested by @lebedev.ri


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D91343/new/

https://reviews.llvm.org/D91343

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
  llvm/test/Transforms/InstCombine/and.ll


Index: llvm/test/Transforms/InstCombine/and.ll
===================================================================
--- llvm/test/Transforms/InstCombine/and.ll
+++ llvm/test/Transforms/InstCombine/and.ll
@@ -979,7 +979,7 @@
 ; CHECK-NEXT:    [[L:%.*]] = shl i32 [[X:%.*]], 20
 ; CHECK-NEXT:    [[R:%.*]] = ashr exact i32 [[L]], 20
 ; CHECK-NEXT:    call void @use32(i32 [[R]])
-; CHECK-NEXT:    [[AND:%.*]] = and i32 [[R]], 4095
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[X]], 4095
 ; CHECK-NEXT:    ret i32 [[AND]]
 ;
   %l = shl i32 %x, 20
@@ -989,6 +989,8 @@
   ret i32 %and
 }
 
+; Negative test - mismatched shift amounts
+
 define i32 @lowmask_not_sext_in_reg(i32 %x) {
 ; CHECK-LABEL: @lowmask_not_sext_in_reg(
 ; CHECK-NEXT:    [[L:%.*]] = shl i32 [[X:%.*]], 19
@@ -1004,6 +1006,8 @@
   ret i32 %and
 }
 
+; Negative test - too much shift for mask
+
 define i32 @not_lowmask_sext_in_reg(i32 %x) {
 ; CHECK-LABEL: @not_lowmask_sext_in_reg(
 ; CHECK-NEXT:    [[L:%.*]] = shl i32 [[X:%.*]], 20
@@ -1019,6 +1023,8 @@
   ret i32 %and
 }
 
+; Negative test - too much shift for mask
+
 define i32 @not_lowmask_sext_in_reg2(i32 %x) {
 ; CHECK-LABEL: @not_lowmask_sext_in_reg2(
 ; CHECK-NEXT:    [[L:%.*]] = shl i32 [[X:%.*]], 21
@@ -1039,7 +1045,7 @@
 ; CHECK-NEXT:    [[L:%.*]] = shl <2 x i32> [[X:%.*]], <i32 20, i32 20>
 ; CHECK-NEXT:    [[R:%.*]] = ashr exact <2 x i32> [[L]], <i32 20, i32 20>
 ; CHECK-NEXT:    store <2 x i32> [[R]], <2 x i32>* [[P:%.*]], align 8
-; CHECK-NEXT:    [[AND:%.*]] = and <2 x i32> [[R]], <i32 4095, i32 4095>
+; CHECK-NEXT:    [[AND:%.*]] = and <2 x i32> [[X]], <i32 4095, i32 4095>
 ; CHECK-NEXT:    ret <2 x i32> [[AND]]
 ;
   %l = shl <2 x i32> %x, <i32 20, i32 20>
Index: llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -899,6 +899,24 @@
 
     break;
   }
+  case Instruction::AShr: {
+    // If the right shift operand 0 is a result of a left shift by the same
+    // amount, this is probably a zero/sign extension, which may be unnecessary,
+    // if we do not demand any of the new sign bits. So, return the original
+    // operand instead.
+    const APInt *ShiftRC;
+    const APInt *ShiftLC;
+    Value *X;
+    unsigned BitWidth = DemandedMask.getBitWidth();
+    if (match(I,
+              m_AShr(m_Shl(m_Value(X), m_APInt(ShiftLC)), m_APInt(ShiftRC))) &&
+        ShiftLC == ShiftRC &&
+        DemandedMask.isSubsetOf(APInt::getLowBitsSet(
+            BitWidth, BitWidth - ShiftRC->getZExtValue()))) {
+      return X;
+    }
+    break;
+  }
   default:
     // Compute the Known bits to simplify things downstream.
     computeKnownBits(I, Known, Depth, CxtI);


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