[llvm] 2760a80 - [InstCombine] dropRedundantMaskingOfLeftShiftInput(): check that adding shift amounts doesn't overflow (PR49778)

[Roman Lebedev via llvm-commits]

Author: Roman Lebedev
Date: 2021-04-04T23:26:41+03:00
New Revision: 2760a808b9916a2839513b7fd7314a464f52481e

URL: https://github.com/llvm/llvm-project/commit/2760a808b9916a2839513b7fd7314a464f52481e
DIFF: https://github.com/llvm/llvm-project/commit/2760a808b9916a2839513b7fd7314a464f52481e.diff

LOG: [InstCombine] dropRedundantMaskingOfLeftShiftInput(): check that adding shift amounts doesn't overflow (PR49778)

This is identical to 781d077afb0ed9771c513d064c40170c1ccd21c9,
but for the other function.

For certain shift amount bit widths, we must first ensure that adding
shift amounts is safe, that the sum won't have an unsigned overflow.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49778

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
    llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-pr49778.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 8413ae95c29e..522374017181 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -226,9 +226,9 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
     // Peek through an optional zext of the shift amount.
     match(MaskShAmt, m_ZExtOrSelf(m_Value(MaskShAmt)));
 
-    // We have two shift amounts from two 
diff erent shifts. The types of those
-    // shift amounts may not match. If that's the case let's bailout now.
-    if (MaskShAmt->getType() != ShiftShAmt->getType())
+    // Verify that it would be safe to try to add those two shift amounts.
+    if (!canTryToConstantAddTwoShiftAmounts(OuterShift, ShiftShAmt, Masked,
+                                            MaskShAmt))
       return nullptr;
 
     // Can we simplify (MaskShAmt+ShiftShAmt) ?
@@ -258,9 +258,9 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
     // Peek through an optional zext of the shift amount.
     match(MaskShAmt, m_ZExtOrSelf(m_Value(MaskShAmt)));
 
-    // We have two shift amounts from two 
diff erent shifts. The types of those
-    // shift amounts may not match. If that's the case let's bailout now.
-    if (MaskShAmt->getType() != ShiftShAmt->getType())
+    // Verify that it would be safe to try to add those two shift amounts.
+    if (!canTryToConstantAddTwoShiftAmounts(OuterShift, ShiftShAmt, Masked,
+                                            MaskShAmt))
       return nullptr;
 
     // Can we simplify (ShiftShAmt-MaskShAmt) ?

diff  --git a/llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-pr49778.ll b/llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-pr49778.ll
index 4865afa56a03..8d70733ed03d 100644
--- a/llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-pr49778.ll
+++ b/llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-pr49778.ll
@@ -4,7 +4,12 @@
 ; PR49778: this should not be folded to 0.
 define i32 @src(i1 %x2) {
 ; CHECK-LABEL: @src(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    [[X13:%.*]] = zext i1 [[X2:%.*]] to i32
+; CHECK-NEXT:    [[_7:%.*]] = shl i32 -1, [[X13]]
+; CHECK-NEXT:    [[MASK:%.*]] = xor i32 [[_7]], -1
+; CHECK-NEXT:    [[_8:%.*]] = and i32 [[MASK]], [[X13]]
+; CHECK-NEXT:    [[_9:%.*]] = shl i32 [[_8]], [[X13]]
+; CHECK-NEXT:    ret i32 [[_9]]
 ;
   %x13 = zext i1 %x2 to i32
   %_7 = shl i32 4294967295, %x13