cbe9891b44
Resolves: https://github.com/llvm/llvm-project/issues/164150 C++26 allows for constexpr packs in structured bindings. This is a new feature (the code doesn't compile on lower the -std=c++26) and so was previously unhandled in clang. This makes clang aware of packs and handle them as one constant unit instead of materializing them as separate mutable reference temporaries allowing llvm to optimize them. This turns the example code from the issue into this as you would expect without compiling for zen 5 (the good codegen described). ```asm movq %rdi, %rax movups (%rsi), %xmm0 movups %xmm0, (%rdi) movups (%rdx), %xmm0 movups %xmm0, 16(%rdi) retq ```
30 lines
1.1 KiB
C++
30 lines
1.1 KiB
C++
// Tests lifetime-extended temporaries with mutable subobjects are always
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// emitted as `internal global`, never `internal constant` even for:
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// - `const` references with an initializer to a const expression,
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// - or binding to a mutable member of an otherwise const object.
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//
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// Mutable members can be modified through a const object, so they can't be
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// placed in read-only memory.
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//
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// RUN: %clang_cc1 -std=c++11 -triple x86_64-linux-gnu -emit-llvm -o - %s | FileCheck %s
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// RUN: %clang_cc1 -std=c++17 -triple x86_64-linux-gnu -emit-llvm -o - %s | FileCheck %s
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struct WithMutable {
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int x;
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mutable int m;
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constexpr WithMutable(int X, int M) : x(X), m(M) {}
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};
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const WithMutable &direct_ref = WithMutable(1, 2);
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void touch_direct() { direct_ref.m = 7; }
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// CHECK: @_ZGR10direct_ref_ = internal global %struct.WithMutable
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struct MutableMember {
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mutable int m;
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constexpr MutableMember(int v) : m(v) {}
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};
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const int &member_ref = MutableMember(5).m;
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void write_member() { const_cast<int &>(member_ref) = 9; }
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// CHECK: @_ZGR10member_ref_ = internal global %struct.MutableMember
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