e2d7cd685d
This patch makes the dead_on_return parameter attribute optionally require a number
of bytes to be passed in to specify the number of bytes known to be dead
upon function return/unwind. This is aimed at enabling annotating the
this pointer in C++ destructors with dead_on_return in clang. We need
this to handle cases like the following:
```
struct X {
int n;
~X() {
this[n].n = 0;
}
};
void f() {
X xs[] = {42, -1};
}
```
Where we only certain that sizeof(X) bytes are dead upon return of ~X.
Otherwise DSE would be able to eliminate the store in ~X which would not
be correct.
This patch only does the wiring within IR. Future patches will make
clang emit correct sizing information and update DSE to only delete
stores to objects marked dead_on_return that are provably in bounds of
the number of bytes specified to be dead_on_return.
Reviewers: nikic, alinas, antoniofrighetto
Pull Request: https://github.com/llvm/llvm-project/pull/171712
74 lines
2.4 KiB
C++
74 lines
2.4 KiB
C++
// RUN: %clang_cc1 -fpass-by-value-is-noalias -triple arm64-apple-iphoneos -emit-llvm -disable-llvm-optzns %s -o - 2>&1 | FileCheck --check-prefix=WITH_NOALIAS %s
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// RUN: %clang_cc1 -triple arm64-apple-iphoneos -emit-llvm -disable-llvm-optzns %s -o - 2>&1 | FileCheck --check-prefix=NO_NOALIAS %s
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// A trivial struct large enough so it is not passed in registers on ARM64.
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struct Foo {
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int a;
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int b;
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int c;
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int d;
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int e;
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int f;
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};
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// Make sure noalias is added to indirect arguments with trivially copyable types
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// if -fpass-by-value-is-noalias is provided.
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// WITH_NOALIAS: define{{.*}} void @_Z4take3Foo(ptr noalias noundef dead_on_return %arg)
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// NO_NOALIAS: define{{.*}} void @_Z4take3Foo(ptr noundef dead_on_return %arg)
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void take(Foo arg) {}
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int G;
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// NonTrivial is not trivially-copyable, because it has a non-trivial copy
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// constructor.
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struct NonTrivial {
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int a;
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int b;
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int c;
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int d;
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int e;
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int f;
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NonTrivial(const NonTrivial &Other) {
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a = G + 10 + Other.a;
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}
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};
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// Make sure noalias is not added to indirect arguments that are not trivially
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// copyable even if -fpass-by-value-is-noalias is provided.
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// WITH_NOALIAS: define{{.*}} void @_Z4take10NonTrivial(ptr noundef dead_on_return %arg)
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// NO_NOALIAS: define{{.*}} void @_Z4take10NonTrivial(ptr noundef dead_on_return %arg)
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void take(NonTrivial arg) {}
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// Escape examples. Pointers to the objects passed to take() may escape, depending on whether a temporary copy is created or not (e.g. due to NRVO).
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struct A {
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A(A **where) : data{"hello world 1"} {
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*where = this; //Escaped pointer 1 (proposed UB?)
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}
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A() : data{"hello world 2"} {}
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char data[32];
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};
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A *p;
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// WITH_NOALIAS: define{{.*}} void @_Z4take1A(ptr noalias noundef dead_on_return %arg)
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// NO_NOALIAS: define{{.*}} void @_Z4take1A(ptr noundef dead_on_return %arg)
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void take(A arg) {}
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// WITH_NOALIAS: define{{.*}} void @_Z7CreateAPP1A(ptr dead_on_unwind noalias writable sret(%struct.A) align 1 %agg.result, ptr noundef %where)
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// NO_NOALIAS: define{{.*}} void @_Z7CreateAPP1A(ptr dead_on_unwind noalias writable sret(%struct.A) align 1 %agg.result, ptr noundef %where)
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A CreateA(A **where) {
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A justlikethis;
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*where = &justlikethis; //Escaped pointer 2 (should also be UB, then)
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return justlikethis;
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}
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// elsewhere, perhaps compiled by a smarter compiler that doesn't make a copy here
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void test() {
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take({&p}); // 1
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take(CreateA(&p)); // 2
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}
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