llvm · nikic · Dec 14, 2023 · Aug 9, 2023 · Dec 7, 2023
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
@@ -1554,6 +1554,20 @@ Currently, only the following parameter attributes are defined:
     ``readonly`` or a ``memory`` attribute that does not contain
     ``argmem: write``.
 
+``dead_on_unwind``
+    At a high level, this attribute indicates that the pointer argument is dead
+    if the call unwinds, in the sense that the caller will not depend on the
+    contents of the memory. Stores that would only be visible on the unwind
+    path can be elided.
+
+    More precisely, the behavior is as-if any memory written through the
+    pointer during the execution of the function is overwritten with a poison
+    value on unwind. This includes memory written by the implicit write implied
+    by the ``writable`` attribute. The caller is allowed to access the affected
+    memory, but all loads that are not preceded by a store will return poison.
+
+    This attribute cannot be applied to return values.
+
 .. _gc:
 
 Garbage Collector Strategy Names

diff --git a/llvm/include/llvm/Bitcode/LLVMBitCodes.h b/llvm/include/llvm/Bitcode/LLVMBitCodes.h
@@ -723,6 +723,7 @@ enum AttributeKindCodes {
   ATTR_KIND_OPTIMIZE_FOR_DEBUGGING = 88,
   ATTR_KIND_WRITABLE = 89,
   ATTR_KIND_CORO_ONLY_DESTROY_WHEN_COMPLETE = 90,
+  ATTR_KIND_DEAD_ON_UNWIND = 91,
 };
 
 enum ComdatSelectionKindCodes {

diff --git a/llvm/include/llvm/IR/Attributes.td b/llvm/include/llvm/IR/Attributes.td
@@ -151,6 +151,9 @@ def NoDuplicate : EnumAttr<"noduplicate", [FnAttr]>;
 /// Function does not deallocate memory.
 def NoFree : EnumAttr<"nofree", [FnAttr, ParamAttr]>;
 
+/// Argument is dead if the call unwinds.
+def DeadOnUnwind : EnumAttr<"dead_on_unwind", [ParamAttr]>;
+
 /// Disable implicit floating point insts.
 def NoImplicitFloat : EnumAttr<"noimplicitfloat", [FnAttr]>;
 

diff --git a/llvm/lib/Analysis/AliasAnalysis.cpp b/llvm/lib/Analysis/AliasAnalysis.cpp
@@ -896,7 +896,7 @@ bool llvm::isNotVisibleOnUnwind(const Value *Object,
 
   // Byval goes out of scope on unwind.
   if (auto *A = dyn_cast<Argument>(Object))
-    return A->hasByValAttr();
+    return A->hasByValAttr() || A->hasAttribute(Attribute::DeadOnUnwind);
 
   // A noalias return is not accessible from any other code. If the pointer
   // does not escape prior to the unwind, then the caller cannot access the

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -2065,6 +2065,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::Writable;
   case bitc::ATTR_KIND_CORO_ONLY_DESTROY_WHEN_COMPLETE:
     return Attribute::CoroDestroyOnlyWhenComplete;
+  case bitc::ATTR_KIND_DEAD_ON_UNWIND:
+    return Attribute::DeadOnUnwind;
   }
 }
 

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -827,6 +827,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_WRITABLE;
   case Attribute::CoroDestroyOnlyWhenComplete:
     return bitc::ATTR_KIND_CORO_ONLY_DESTROY_WHEN_COMPLETE;
+  case Attribute::DeadOnUnwind:
+    return bitc::ATTR_KIND_DEAD_ON_UNWIND;
   case Attribute::EndAttrKinds:
     llvm_unreachable("Can not encode end-attribute kinds marker.");
   case Attribute::None:

diff --git a/llvm/lib/IR/Attributes.cpp b/llvm/lib/IR/Attributes.cpp
@@ -1962,7 +1962,8 @@ AttributeMask AttributeFuncs::typeIncompatible(Type *Ty,
           .addAttribute(Attribute::ReadOnly)
           .addAttribute(Attribute::Dereferenceable)
           .addAttribute(Attribute::DereferenceableOrNull)
-          .addAttribute(Attribute::Writable);
+          .addAttribute(Attribute::Writable)
+          .addAttribute(Attribute::DeadOnUnwind);
     if (ASK & ASK_UNSAFE_TO_DROP)
       Incompatible.addAttribute(Attribute::Nest)
           .addAttribute(Attribute::SwiftError)

diff --git a/llvm/lib/Transforms/Utils/CodeExtractor.cpp b/llvm/lib/Transforms/Utils/CodeExtractor.cpp
@@ -998,6 +998,7 @@ Function *CodeExtractor::constructFunction(const ValueSet &inputs,
       case Attribute::ByRef:
       case Attribute::WriteOnly:
       case Attribute::Writable:
+      case Attribute::DeadOnUnwind:
       //  These are not really attributes.
       case Attribute::None:
       case Attribute::EndAttrKinds:

diff --git a/llvm/test/Bitcode/attributes.ll b/llvm/test/Bitcode/attributes.ll
@@ -521,6 +521,11 @@ define void @f90(ptr writable %p) {
   ret void
 }
 
+; CHECK: define void @f91(ptr dead_on_unwind %p)
+define void @f91(ptr dead_on_unwind %p) {
+  ret void
+}
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { memory(none) }

diff --git a/llvm/test/Transforms/DeadStoreElimination/simple.ll b/llvm/test/Transforms/DeadStoreElimination/simple.ll
@@ -514,13 +514,12 @@ define void @test34(ptr noalias %p) {
   store i32 0, ptr %p
   ret void
 }
-; Same as previous case, but with an sret argument.
-; TODO: The first store could be eliminated if sret is not visible on unwind.
-define void @test34_sret(ptr noalias sret(i32) %p) {
-; CHECK-LABEL: @test34_sret(
-; CHECK-NEXT:    store i32 1, ptr [[P:%.*]], align 4
+
+; Same as previous case, but with a dead_on_unwind argument.
+define void @test34_dead_on_unwind(ptr noalias dead_on_unwind %p) {
+; CHECK-LABEL: @test34_dead_on_unwind(
 ; CHECK-NEXT:    call void @unknown_func()
-; CHECK-NEXT:    store i32 0, ptr [[P]], align 4
+; CHECK-NEXT:    store i32 0, ptr [[P:%.*]], align 4
 ; CHECK-NEXT:    ret void
 ;
   store i32 1, ptr %p

diff --git a/llvm/test/Transforms/LICM/scalar-promote-unwind.ll b/llvm/test/Transforms/LICM/scalar-promote-unwind.ll
@@ -147,17 +147,15 @@ for.cond.cleanup:
   ret void
 }
 
-; TODO: sret could be specified to not be accessed on unwind either.
-define void @test_sret(ptr noalias sret(i32) %a, i1 zeroext %y) uwtable {
-; CHECK-LABEL: @test_sret(
+define void @test_dead_on_unwind(ptr noalias dead_on_unwind %a, i1 zeroext %y) uwtable {
+; CHECK-LABEL: @test_dead_on_unwind(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[A_PROMOTED:%.*]] = load i32, ptr [[A:%.*]], align 4
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[ADD1:%.*]] = phi i32 [ [[A_PROMOTED]], [[ENTRY:%.*]] ], [ [[ADD:%.*]], [[FOR_INC:%.*]] ]
 ; CHECK-NEXT:    [[I_03:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[INC:%.*]], [[FOR_INC]] ]
 ; CHECK-NEXT:    [[ADD]] = add nsw i32 [[ADD1]], 1
-; CHECK-NEXT:    store i32 [[ADD]], ptr [[A]], align 4
 ; CHECK-NEXT:    br i1 [[Y:%.*]], label [[IF_THEN:%.*]], label [[FOR_INC]]
 ; CHECK:       if.then:
 ; CHECK-NEXT:    tail call void @f()
@@ -167,6 +165,8 @@ define void @test_sret(ptr noalias sret(i32) %a, i1 zeroext %y) uwtable {
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i32 [[INC]], 10000
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_COND_CLEANUP:%.*]], label [[FOR_BODY]]
 ; CHECK:       for.cond.cleanup:
+; CHECK-NEXT:    [[ADD_LCSSA:%.*]] = phi i32 [ [[ADD]], [[FOR_INC]] ]
+; CHECK-NEXT:    store i32 [[ADD_LCSSA]], ptr [[A]], align 4
 ; CHECK-NEXT:    ret void
 ;
 entry:

diff --git a/llvm/test/Transforms/MemCpyOpt/callslot_throw.ll b/llvm/test/Transforms/MemCpyOpt/callslot_throw.ll
@@ -56,9 +56,39 @@ entry:
   ret void
 }
 
-; TODO: With updated semantics, sret could also be invisible on unwind.
-define void @test_sret(ptr nocapture noalias dereferenceable(4) sret(i32) %x) {
-; CHECK-LABEL: @test_sret(
+define void @test_dead_on_unwind(ptr nocapture noalias writable dead_on_unwind dereferenceable(4) %x) {
+; CHECK-LABEL: @test_dead_on_unwind(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[T:%.*]] = alloca i32, align 4
+; CHECK-NEXT:    call void @may_throw(ptr nonnull [[X:%.*]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  %t = alloca i32, align 4
+  call void @may_throw(ptr nonnull %t)
+  %load = load i32, ptr %t, align 4
+  store i32 %load, ptr %x, align 4
+  ret void
+}
+
+; Same as previous test, but dereferenceability information is provided by sret.
+define void @test_dead_on_unwind_sret(ptr nocapture noalias writable dead_on_unwind sret(i32) %x) {
+; CHECK-LABEL: @test_dead_on_unwind_sret(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[T:%.*]] = alloca i32, align 4
+; CHECK-NEXT:    call void @may_throw(ptr nonnull [[X:%.*]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  %t = alloca i32, align 4
+  call void @may_throw(ptr nonnull %t)
+  %load = load i32, ptr %t, align 4
+  store i32 %load, ptr %x, align 4
+  ret void
+}
+
+define void @test_dead_on_unwind_missing_writable(ptr nocapture noalias dead_on_unwind dereferenceable(4) %x) {
+; CHECK-LABEL: @test_dead_on_unwind_missing_writable(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[T:%.*]] = alloca i32, align 4
 ; CHECK-NEXT:    call void @may_throw(ptr nonnull [[T]])

diff --git a/llvm/test/Verifier/dead-on-unwind.ll b/llvm/test/Verifier/dead-on-unwind.ll
@@ -0,0 +1,7 @@
+; RUN: not llvm-as -disable-output %s 2>&1 | FileCheck %s
+
+; CHECK: Attribute 'dead_on_unwind' applied to incompatible type!
+; CHECK-NEXT: ptr @not_pointer
+define void @not_pointer(i32 dead_on_unwind %arg) {
+  ret void
+}