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Differential D158984

[Clang] Allow __declspec(noalias) to access inaccessible memory
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Authored by nikic on Aug 28 2023, 6:41 AM.

Details

Reviewers
rnk
Commits
rG14cc7a077275: [Clang] Allow __declspec(noalias) to access inaccessible memory
Summary

MSVC defines __declspec(noalias) as follows (https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2012/k649tyc7(v=vs.110)?redirectedfrom=MSDN):

noalias means that a function call does not modify or reference visible global state and only modifies the memory pointed to directly by pointer parameters (first-level indirections).

If a function is annotated as noalias, the optimizer can assume that, in addition to the parameters themselves, only first-level indirections of pointer parameters are referenced or modified inside the function. The visible global state is the set of all data that is not defined or referenced outside of the compilation scope, and their address is not taken. The compilation scope is all source files (/LTCG (Link-time Code Generation) builds) or a single source file (non-/LTCG build).

The wording is not super clear to me, but I believe this is saying that __declspec(noalias) functions may access inaccessible memory (i.e. non-visible global state in their words). Indeed, the Windows CRT applies this attribute to malloc, which does access inaccessible memory under LLVM's memory model.

As such, change the attribute to emit memory(argmem: readwrite, inaccessiblemem: readwrite) instead of memory(argmem: readwrite).

Fixes https://github.com/llvm/llvm-project/issues/64827.

Diff Detail

Event Timeline

nikic created this revision.Aug 28 2023, 6:41 AM
Herald added a project: Restricted Project. · View Herald TranscriptAug 28 2023, 6:41 AM
Herald added subscribers: jeroen.dobbelaere, StephenFan. · View Herald Transcript
nikic requested review of this revision.Aug 28 2023, 6:41 AM
Harbormaster completed remote builds in B255208: Diff 553914.Aug 28 2023, 7:56 AM
rnk accepted this revision.Aug 28 2023, 2:46 PM

lgtm

This revision is now accepted and ready to land.Aug 28 2023, 2:46 PM
Closed by commit rG14cc7a077275: [Clang] Allow __declspec(noalias) to access inaccessible memory (authored by nikic). · Explain WhyAug 29 2023, 2:44 AM
This revision was automatically updated to reflect the committed changes.
nikic added a commit: rG14cc7a077275: [Clang] Allow __declspec(noalias) to access inaccessible memory.
Herald added a project: Restricted Project. · View Herald TranscriptAug 29 2023, 2:44 AM
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Revision Contents

PathSize
clang/
lib/
CodeGen/
2 lines
test/
CodeGen/
2 lines

Diff 554229

clang/lib/CodeGen/CGCall.cpp

Show First 20 Lines • Show All 2,369 Lines • ▼ Show 20 Lines if (TargetDecl->hasAttr<ConstAttr>()) {
// 'pure' functions, so they also cannot have infinite loops. // 'pure' functions, so they also cannot have infinite loops.
FuncAttrs.addAttribute(llvm::Attribute::WillReturn); FuncAttrs.addAttribute(llvm::Attribute::WillReturn);
} else if (TargetDecl->hasAttr<PureAttr>()) { } else if (TargetDecl->hasAttr<PureAttr>()) {
FuncAttrs.addMemoryAttr(llvm::MemoryEffects::readOnly()); FuncAttrs.addMemoryAttr(llvm::MemoryEffects::readOnly());
FuncAttrs.addAttribute(llvm::Attribute::NoUnwind); FuncAttrs.addAttribute(llvm::Attribute::NoUnwind);
// gcc specifies that 'pure' functions cannot have infinite loops. // gcc specifies that 'pure' functions cannot have infinite loops.
FuncAttrs.addAttribute(llvm::Attribute::WillReturn); FuncAttrs.addAttribute(llvm::Attribute::WillReturn);
} else if (TargetDecl->hasAttr<NoAliasAttr>()) { } else if (TargetDecl->hasAttr<NoAliasAttr>()) {
FuncAttrs.addMemoryAttr(llvm::MemoryEffects::argMemOnly()); FuncAttrs.addMemoryAttr(llvm::MemoryEffects::inaccessibleOrArgMemOnly());
FuncAttrs.addAttribute(llvm::Attribute::NoUnwind); FuncAttrs.addAttribute(llvm::Attribute::NoUnwind);
} }
if (TargetDecl->hasAttr<RestrictAttr>()) if (TargetDecl->hasAttr<RestrictAttr>())
RetAttrs.addAttribute(llvm::Attribute::NoAlias); RetAttrs.addAttribute(llvm::Attribute::NoAlias);
if (TargetDecl->hasAttr<ReturnsNonNullAttr>() && if (TargetDecl->hasAttr<ReturnsNonNullAttr>() &&
!CodeGenOpts.NullPointerIsValid) !CodeGenOpts.NullPointerIsValid)
RetAttrs.addAttribute(llvm::Attribute::NonNull); RetAttrs.addAttribute(llvm::Attribute::NonNull);
if (TargetDecl->hasAttr<AnyX86NoCallerSavedRegistersAttr>()) if (TargetDecl->hasAttr<AnyX86NoCallerSavedRegistersAttr>())
▲ Show 20 Lines • Show All 3,500 Lines • Show Last 20 Lines

clang/test/CodeGen/ms-declspecs.c

Show All 35 Lines
__declspec(noalias) void noalias_callee(int *x); __declspec(noalias) void noalias_callee(int *x);
// CHECK: call void @noalias_callee({{.*}}) [[NA:#[0-9]+]] // CHECK: call void @noalias_callee({{.*}}) [[NA:#[0-9]+]]
void noalias_caller(int *x) { noalias_callee(x); } void noalias_caller(int *x) { noalias_callee(x); }
// CHECK: attributes [[NAKED]] = { naked noinline nounwind{{.*}} } // CHECK: attributes [[NAKED]] = { naked noinline nounwind{{.*}} }
// CHECK: attributes [[NUW]] = { nounwind{{.*}} } // CHECK: attributes [[NUW]] = { nounwind{{.*}} }
// CHECK: attributes [[NI]] = { noinline nounwind{{.*}} } // CHECK: attributes [[NI]] = { noinline nounwind{{.*}} }
// CHECK: attributes [[NR]] = { noreturn } // CHECK: attributes [[NR]] = { noreturn }
// CHECK: attributes [[NA]] = { nounwind memory(argmem: readwrite){{.*}} } // CHECK: attributes [[NA]] = { nounwind memory(argmem: readwrite, inaccessiblemem: readwrite){{.*}} }