[llvm] 1206a18 - [InstCombine] guard against splat-mul corner case

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2022-04-11T15:50:13-04:00
New Revision: 1206a18d417aa6d9d3d6f9e25cbf0b07ed1409a4

URL: https://github.com/llvm/llvm-project/commit/1206a18d417aa6d9d3d6f9e25cbf0b07ed1409a4
DIFF: https://github.com/llvm/llvm-project/commit/1206a18d417aa6d9d3d6f9e25cbf0b07ed1409a4.diff

LOG: [InstCombine] guard against splat-mul corner case

The test is already simplified, and I'm not sure how
to write a test to exercise the new clause. But it
protects the 2-bit pattern from miscompiling as noted
in D123453.

https://alive2.llvm.org/ce/z/QPyVfv
(If we managed to fall into the mul transform, it
would wrongly create a zero on this pattern.)

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
    llvm/test/Transforms/InstCombine/lshr.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index b38ee71e7bf67..26fe8e08d066d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -1165,11 +1165,11 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
 
     // 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 i32 (mul nuw X, Pow2+1), 16 --> and X, Pow2-1
+    // 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))) &&
-        ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
+        BitWidth > 2 && ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
         MulC->logBase2() == ShAmtC)
       return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
 

diff  --git a/llvm/test/Transforms/InstCombine/lshr.ll b/llvm/test/Transforms/InstCombine/lshr.ll
index 2db2c09911769..e4e869b921edd 100644
--- a/llvm/test/Transforms/InstCombine/lshr.ll
+++ b/llvm/test/Transforms/InstCombine/lshr.ll
@@ -399,6 +399,17 @@ define i32 @mul_splat_fold_no_nuw(i32 %x) {
   ret i32 %t
 }
 
+; Negative test (but simplifies before we reach the mul_splat transform)- need more than 2 bits
+
+define i2 @mul_splat_fold_too_narrow(i2 %x) {
+; CHECK-LABEL: @mul_splat_fold_too_narrow(
+; CHECK-NEXT:    ret i2 [[X:%.*]]
+;
+  %m = mul nuw i2 %x, 2
+  %t = lshr i2 %m, 1
+  ret i2 %t
+}
+
 define i32 @negative_and_odd(i32 %x) {
 ; CHECK-LABEL: @negative_and_odd(
 ; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 [[X:%.*]], 31