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Summary: Right now we annotate C++'s `operator new` with `noalias` attribute, which very much is healthy for optimizations. However as per [[ http://eel.is/c++draft/basic.stc.dynamic.allocation | `[basic.stc.dynamic.allocation]` ]], there are more promises on global `operator new`, namely: * non-`std::nothrow_t` `operator new` *never* returns `nullptr` * If `std::align_val_t align` parameter is taken, the pointer will also be `align`-aligned * ~~global `operator new`-returned pointer is `__STDCPP_DEFAULT_NEW_ALIGNMENT__`-aligned ~~ It's more caveated than that. Supplying this information may not cause immediate landslide effects on any specific benchmarks, but it for sure will be healthy for optimizer in the sense that the IR will better reflect the guarantees provided in the source code. The caveat is `-fno-assume-sane-operator-new`, which currently prevents emitting `noalias` attribute, and is automatically passed by Sanitizers ([[ https://bugs.llvm.org/show_bug.cgi?id=16386 | PR16386 ]]) - should it also cover these attributes? The problem is that the flag is back-end-specific, as seen in `test/Modules/explicit-build-flags.cpp`. But while it is okay to add `noalias` metadata in backend, we really should be adding at least the alignment metadata to the AST, since that allows us to perform sema checks on it. Reviewers: erichkeane, rjmccall, jdoerfert, eugenis, rsmith Reviewed By: rsmith Subscribers: xbolva00, jrtc27, atanasyan, nlopes, cfe-commits Tags: #llvm, #clang Differential Revision: https://reviews.llvm.org/D73380
72 lines
2.3 KiB
C++
72 lines
2.3 KiB
C++
// RUN: %clang_cc1 -emit-llvm %s -o - -triple=i386-pc-win32 | FileCheck %s
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struct ClassWithoutDtor {
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char x;
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};
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void check_array_no_cookies() {
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// CHECK: define dso_local void @"?check_array_no_cookies@@YAXXZ"() [[NUW:#[0-9]+]]
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// CHECK: call noalias nonnull i8* @"??_U@YAPAXI@Z"(i32 42)
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ClassWithoutDtor *array = new ClassWithoutDtor[42];
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// CHECK: call void @"??_V@YAXPAX@Z"(
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delete [] array;
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}
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struct ClassWithDtor {
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char x;
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~ClassWithDtor() {}
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};
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void check_array_cookies_simple() {
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// CHECK: define {{.*}} @"?check_array_cookies_simple@@YAXXZ"()
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ClassWithDtor *array = new ClassWithDtor[42];
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// CHECK: [[ALLOCATED:%.*]] = call noalias nonnull i8* @"??_U@YAPAXI@Z"(i32 46)
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// 46 = 42 + size of cookie (4)
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// CHECK: [[COOKIE:%.*]] = bitcast i8* [[ALLOCATED]] to i32*
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// CHECK: store i32 42, i32* [[COOKIE]]
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// CHECK: [[ARRAY:%.*]] = getelementptr inbounds i8, i8* [[ALLOCATED]], i32 4
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// CHECK: bitcast i8* [[ARRAY]] to [[CLASS:%.*]]*
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delete [] array;
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// CHECK: [[ARRAY_AS_CHAR:%.*]] = bitcast [[CLASS]]* {{%.*}} to i8*
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// CHECK: getelementptr inbounds i8, i8* [[ARRAY_AS_CHAR]], i32 -4
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}
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struct __attribute__((aligned(8))) ClassWithAlignment {
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// FIXME: replace __attribute__((aligned(8))) with __declspec(align(8)) once
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// http://llvm.org/bugs/show_bug.cgi?id=12631 is fixed.
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int *x, *y;
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~ClassWithAlignment() {}
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};
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void check_array_cookies_aligned() {
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// CHECK: define {{.*}} @"?check_array_cookies_aligned@@YAXXZ"()
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ClassWithAlignment *array = new ClassWithAlignment[42];
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// CHECK: [[ALLOCATED:%.*]] = call noalias nonnull i8* @"??_U@YAPAXI@Z"(i32 344)
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// 344 = 42*8 + size of cookie (8, due to alignment)
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// CHECK: [[COOKIE:%.*]] = bitcast i8* [[ALLOCATED]] to i32*
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// CHECK: store i32 42, i32* [[COOKIE]]
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// CHECK: [[ARRAY:%.*]] = getelementptr inbounds i8, i8* [[ALLOCATED]], i32 8
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// CHECK: bitcast i8* [[ARRAY]] to [[CLASS:%.*]]*
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delete [] array;
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// CHECK: [[ARRAY_AS_CHAR:%.*]] = bitcast [[CLASS]]*
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// CHECK: getelementptr inbounds i8, i8* [[ARRAY_AS_CHAR]], i32 -8
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}
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namespace PR23990 {
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struct S {
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char x[42];
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void operator delete[](void *p, __SIZE_TYPE__);
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// CHECK-LABEL: define dso_local void @"?delete_s@PR23990@@YAXPAUS@1@@Z"(
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// CHECK: call void @"??_VS@PR23990@@SAXPAXI@Z"(i8* {{.*}}, i32 42)
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};
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void delete_s(S *s) { delete[] s; }
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}
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// CHECK: attributes [[NUW]] = { noinline nounwind{{.*}} }
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