[IR] Handle nofree noalias in canBeFreed()

[nikic]

Based on the argument nofree semantics specified in #195658, we can conclude that an argument with both nofree and noalias cannot be freed.

This also handles the case of readonly + noalias, to be consistent with the logic for functions (and because we had a FIXME for it...)

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@llvm/pr-subscribers-llvm-analysis

@llvm/pr-subscribers-llvm-ir

Author: Nikita Popov (nikic)

Changes

Based on the argument nofree semantics specified in #195658, we can conclude that an argument with both nofree and noalias cannot be freed.

This also handles the case of readonly + noalias, to be consistent with the logic for functions (and because we had a FIXME for it...)

---

Full diff: https://github.com/llvm/llvm-project/pull/200194.diff

2 Files Affected:

• (modified) llvm/lib/IR/Value.cpp (+6)
• (modified) llvm/test/Analysis/ValueTracking/memory-dereferenceable.ll (+18-5)

diff --git a/llvm/lib/IR/Value.cpp b/llvm/lib/IR/Value.cpp
index 7246adf7ec651..439c3572bdccd 100644
--- a/llvm/lib/IR/Value.cpp
+++ b/llvm/lib/IR/Value.cpp
@@ -842,6 +842,12 @@ bool Value::canBeFreed() const {
     const Function *F = A->getParent();
     if (F->doesNotFreeMemory() && F->hasNoSync())
       return false;
+
+    // nofree on the argument ensures that it cannot be freed through that
+    // pointer. noalias additionally ensures that it can't be freed through
+    // another pointer to the same allocation. Readonly implies nofree.
+    if ((A->hasNoFreeAttr() || A->onlyReadsMemory()) && A->hasNoAliasAttr())
+      return false;
   }
 
   if (auto *ITP = dyn_cast<IntToPtrInst>(this);
diff --git a/llvm/test/Analysis/ValueTracking/memory-dereferenceable.ll b/llvm/test/Analysis/ValueTracking/memory-dereferenceable.ll
index fc40c68624f2d..0edd95dbbb2f4 100644
--- a/llvm/test/Analysis/ValueTracking/memory-dereferenceable.ll
+++ b/llvm/test/Analysis/ValueTracking/memory-dereferenceable.ll
@@ -265,9 +265,7 @@ define void @infer_func_attrs2(ptr dereferenceable(8) %p) readonly {
 }
 
 ; CHECK-LABEL: 'infer_noalias1'
-; GLOBAL: %p
-; POINT-NOT: %p
-; FIXME: Can be inferred from attributes
+; CHECK: %p
 define void @infer_noalias1(ptr dereferenceable(8) noalias nofree %p) {
   call void @mayfree()
   %v = load i32, ptr %p
@@ -275,15 +273,30 @@ define void @infer_noalias1(ptr dereferenceable(8) noalias nofree %p) {
 }
 
 ; CHECK-LABEL: 'infer_noalias2'
+; CHECK: %p
+define void @infer_noalias2(ptr dereferenceable(8) noalias readonly %p) nosync {
+  call void @mayfree()
+  %v = load i32, ptr %p
+  ret void
+}
+
+; CHECK-LABEL: 'infer_missing_noalias1'
 ; GLOBAL: %p
 ; POINT-NOT: %p
-; FIXME: Can be inferred from attributes
-define void @infer_noalias2(ptr dereferenceable(8) noalias readonly %p) nosync {
+define void @infer_missing_noalias1(ptr dereferenceable(8) nofree %p) {
   call void @mayfree()
   %v = load i32, ptr %p
   ret void
 }
 
+; CHECK-LABEL: 'infer_missing_noalias2'
+; GLOBAL: %p
+; POINT-NOT: %p
+define void @infer_missing_noalias2(ptr dereferenceable(8) readonly %p) {
+  call void @mayfree()
+  %v = load i32, ptr %p
+  ret void
+}
 
 ; Just check that we don't crash.
 ; CHECK-LABEL: 'opaque_type_crasher'

[antoniofrighetto]

Maybe nit: the other PR may also drop nosync from this, as otherwise now possibly misleading?

[nikic]

Good point, dropped.

[antoniofrighetto]

LGTM