[llvm] 525d00e - [InstCombine] Fix poison propagation in round up alignment fold

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2024-04-18T10:58:15+09:00
New Revision: 525d00e5edc4b83105c6ad518926b174f76c3a1c

URL: https://github.com/llvm/llvm-project/commit/525d00e5edc4b83105c6ad518926b174f76c3a1c
DIFF: https://github.com/llvm/llvm-project/commit/525d00e5edc4b83105c6ad518926b174f76c3a1c.diff

LOG: [InstCombine] Fix poison propagation in round up alignment fold

We can't directly use the high bits value if it is more poisonous
due to poison elements in the masks.

This fixes the issue reported in
https://github.com/llvm/llvm-project/pull/88217#issuecomment-2061034941.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
    llvm/test/Transforms/InstCombine/integer-round-up-pow2-alignment.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 2d78fcee1152d7..41c3ab85940eaf 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2965,7 +2965,8 @@ foldRoundUpIntegerWithPow2Alignment(SelectInst &SI,
     return nullptr;
 
   if (!XBiasedHighBits->hasOneUse()) {
-    if (*BiasCst == *LowBitMaskCst)
+    // We can't directly return XBiasedHighBits if it is more poisonous.
+    if (*BiasCst == *LowBitMaskCst && impliesPoison(XBiasedHighBits, X))
       return XBiasedHighBits;
     return nullptr;
   }

diff  --git a/llvm/test/Transforms/InstCombine/integer-round-up-pow2-alignment.ll b/llvm/test/Transforms/InstCombine/integer-round-up-pow2-alignment.ll
index c7e0553992b909..afd56abf40a507 100644
--- a/llvm/test/Transforms/InstCombine/integer-round-up-pow2-alignment.ll
+++ b/llvm/test/Transforms/InstCombine/integer-round-up-pow2-alignment.ll
@@ -437,14 +437,17 @@ define i8 @t17_oneuse(i8 %x) {
   ret i8 %x.roundedup
 }
 
-; Bias is equal to the alignment-1 (as opposed to alignment),
-; so we can just replace %x.roundedup with %x.biased.highbits
+; Negative test: We can't replace with %x.biased.highbits because it is
+; more poisonous.
 define <2 x i4> @t18_replacement_0b0001(<2 x i4> %x) {
 ; CHECK-LABEL: @t18_replacement_0b0001(
-; CHECK-NEXT:    [[X_BIASED:%.*]] = add <2 x i4> [[X:%.*]], <i4 3, i4 3>
+; CHECK-NEXT:    [[X_LOWBITS:%.*]] = and <2 x i4> [[X:%.*]], <i4 3, i4 3>
+; CHECK-NEXT:    [[X_LOWBITS_ARE_ZERO:%.*]] = icmp eq <2 x i4> [[X_LOWBITS]], zeroinitializer
+; CHECK-NEXT:    [[X_BIASED:%.*]] = add <2 x i4> [[X]], <i4 3, i4 3>
 ; CHECK-NEXT:    [[X_BIASED_HIGHBITS:%.*]] = and <2 x i4> [[X_BIASED]], <i4 -4, i4 poison>
 ; CHECK-NEXT:    call void @use.v2i4(<2 x i4> [[X_BIASED_HIGHBITS]])
-; CHECK-NEXT:    ret <2 x i4> [[X_BIASED_HIGHBITS]]
+; CHECK-NEXT:    [[X_ROUNDEDUP:%.*]] = select <2 x i1> [[X_LOWBITS_ARE_ZERO]], <2 x i4> [[X]], <2 x i4> [[X_BIASED_HIGHBITS]]
+; CHECK-NEXT:    ret <2 x i4> [[X_ROUNDEDUP]]
 ;
   %x.lowbits = and <2 x i4> %x, <i4 3, i4 3>
   %x.lowbits.are.zero = icmp eq <2 x i4> %x.lowbits, <i4 0, i4 0>
@@ -454,12 +457,17 @@ define <2 x i4> @t18_replacement_0b0001(<2 x i4> %x) {
   %x.roundedup = select <2 x i1> %x.lowbits.are.zero, <2 x i4> %x, <2 x i4> %x.biased.highbits
   ret <2 x i4> %x.roundedup
 }
+; Negative test: We can't replace with %x.biased.highbits because it is
+; more poisonous.
 define <2 x i4> @t18_replacement_0b0010(<2 x i4> %x) {
 ; CHECK-LABEL: @t18_replacement_0b0010(
-; CHECK-NEXT:    [[X_BIASED:%.*]] = add <2 x i4> [[X:%.*]], <i4 3, i4 poison>
+; CHECK-NEXT:    [[X_LOWBITS:%.*]] = and <2 x i4> [[X:%.*]], <i4 3, i4 3>
+; CHECK-NEXT:    [[X_LOWBITS_ARE_ZERO:%.*]] = icmp eq <2 x i4> [[X_LOWBITS]], zeroinitializer
+; CHECK-NEXT:    [[X_BIASED:%.*]] = add <2 x i4> [[X]], <i4 3, i4 poison>
 ; CHECK-NEXT:    [[X_BIASED_HIGHBITS:%.*]] = and <2 x i4> [[X_BIASED]], <i4 -4, i4 -4>
 ; CHECK-NEXT:    call void @use.v2i4(<2 x i4> [[X_BIASED_HIGHBITS]])
-; CHECK-NEXT:    ret <2 x i4> [[X_BIASED_HIGHBITS]]
+; CHECK-NEXT:    [[X_ROUNDEDUP:%.*]] = select <2 x i1> [[X_LOWBITS_ARE_ZERO]], <2 x i4> [[X]], <2 x i4> [[X_BIASED_HIGHBITS]]
+; CHECK-NEXT:    ret <2 x i4> [[X_ROUNDEDUP]]
 ;
   %x.lowbits = and <2 x i4> %x, <i4 3, i4 3>
   %x.lowbits.are.zero = icmp eq <2 x i4> %x.lowbits, <i4 0, i4 0>