[llvm-branch-commits] [llvm] c27ad80 - [InstCombine] dropRedundantMaskingOfLeftShiftInput(): check that adding shift amounts doesn't overflow (PR49778)

[Tom Stellard via llvm-branch-commits]

Author: Roman Lebedev
Date: 2021-05-03T16:41:31-07:00
New Revision: c27ad80507bfea35da07681fd4ec9972ca698015

URL: https://github.com/llvm/llvm-project/commit/c27ad80507bfea35da07681fd4ec9972ca698015
DIFF: https://github.com/llvm/llvm-project/commit/c27ad80507bfea35da07681fd4ec9972ca698015.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

(cherry picked from commit 2760a808b9916a2839513b7fd7314a464f52481e)

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 52f064e17820..127bf8080959 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