[PATCH] D126296: [ValueTracking] Loads with !dereferenceable metadata cannot be undef/poison

[Nikita Popov via Phabricator via llvm-commits]

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A load with `!dereferenceable` or `!dereferenceable_or_null` metadata must return a well-defined (non-undef/poison) value. Effectively they imply `!noundef`. This is the same as we do for the `dereferenceable(N)` attribute.

This should fix https://github.com/llvm/llvm-project/issues/55672, or at least the specific case discussed there.


https://reviews.llvm.org/D126296

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Transforms/InstCombine/freeze.ll


Index: llvm/test/Transforms/InstCombine/freeze.ll
===================================================================
--- llvm/test/Transforms/InstCombine/freeze.ll
+++ llvm/test/Transforms/InstCombine/freeze.ll
@@ -834,8 +834,7 @@
 define i8* @freeze_load_dereferenceable(i8** %ptr) {
 ; CHECK-LABEL: @freeze_load_dereferenceable(
 ; CHECK-NEXT:    [[P:%.*]] = load i8*, i8** [[PTR:%.*]], align 8, !dereferenceable !1
-; CHECK-NEXT:    [[P_FR:%.*]] = freeze i8* [[P]]
-; CHECK-NEXT:    ret i8* [[P_FR]]
+; CHECK-NEXT:    ret i8* [[P]]
 ;
   %p = load i8*, i8** %ptr, !dereferenceable !1
   %p.fr = freeze i8* %p
@@ -845,8 +844,7 @@
 define i8* @freeze_load_dereferenceable_or_null(i8** %ptr) {
 ; CHECK-LABEL: @freeze_load_dereferenceable_or_null(
 ; CHECK-NEXT:    [[P:%.*]] = load i8*, i8** [[PTR:%.*]], align 8, !dereferenceable_or_null !1
-; CHECK-NEXT:    [[P_FR:%.*]] = freeze i8* [[P]]
-; CHECK-NEXT:    ret i8* [[P_FR]]
+; CHECK-NEXT:    ret i8* [[P]]
 ;
   %p = load i8*, i8** %ptr, !dereferenceable_or_null !1
   %p.fr = freeze i8* %p
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -5191,7 +5191,9 @@
   }
 
   if (auto *I = dyn_cast<LoadInst>(V))
-    if (I->getMetadata(LLVMContext::MD_noundef))
+    if (I->hasMetadata(LLVMContext::MD_noundef) ||
+        I->hasMetadata(LLVMContext::MD_dereferenceable) ||
+        I->hasMetadata(LLVMContext::MD_dereferenceable_or_null))
       return true;
 
   if (programUndefinedIfUndefOrPoison(V, PoisonOnly))


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