9e0c06d708
Let Clang emit `dead_on_return` attribute on pointer arguments that are passed indirectly, namely, large aggregates that the ABI mandates be passed by value; thus, the parameter is destroyed within the callee. Writes to such arguments are not observable by the caller after the callee returns. This should desirably enable further MemCpyOpt/DSE optimizations. Previous discussion: https://discourse.llvm.org/t/rfc-add-dead-on-return-attribute/86871.
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 dead_on_return noalias noundef %arg)
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// NO_NOALIAS: define{{.*}} void @_Z4take3Foo(ptr dead_on_return noundef %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 dead_on_return noundef %arg)
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// NO_NOALIAS: define{{.*}} void @_Z4take10NonTrivial(ptr dead_on_return noundef %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 dead_on_return noalias noundef %arg)
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// NO_NOALIAS: define{{.*}} void @_Z4take1A(ptr dead_on_return noundef %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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