6e916d0598
PointerUnion stores a fixed-width `ceil(log2(N))`-bit tag in the low bits of the pointer. This works only when every member type provides at least that many low bits — if the least-aligned type doesn't, compilation fails, even though the higher-aligned types may have plenty of spare bits going to waste. Introduce a variable-length escape-encoded tag that exploits the extra low bits of higher-aligned types, analogous to UTF-8: types are grouped into tiers by NumLowBitsAvailable; each non-final tier reserves one code as an escape prefix, and the next tier extends the tag into the newly available bits. This allows PointerUnion to hold more type variants than a fixed-width tag permits. The fixed-width path is used when the minimum alignment already provides enough bits (the common case); the variable-width path activates only when it doesn't, and requires types to be listed in non-decreasing NumLowBitsAvailable order. I need this for https://github.com/llvm/llvm-project/pull/186923 which requires a 6-member PointerUnion in MLIR TypeRange/ValueRange. On 32-bit systems, some members only provide 2 low bits, insufficient for a 3-bit fixed-width tag.
72 lines
1.8 KiB
C++
72 lines
1.8 KiB
C++
#include <cstdio>
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#include "llvm/ADT/PointerUnion.h"
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struct HasVirtual {
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virtual void func() = 0;
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virtual ~HasVirtual() = default;
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};
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struct DerivedWithVirtual : public HasVirtual {
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virtual void func() override;
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virtual ~DerivedWithVirtual() = default;
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};
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void DerivedWithVirtual::func() {}
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struct alignas(8) Z {};
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struct Derived : public Z {};
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// Types for variable-width tag encoding test.
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// 3 x alignof(4) + 2 x alignof(8) requires escape-coded tags because
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// ceil(log2(5)) = 3 > min(NumLowBitsAvailable) = 2.
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template <int I> struct alignas(4) Align4 {};
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template <int I> struct alignas(8) Align8 {};
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int main() {
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int a = 5;
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float f = 4.0;
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Z z;
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Derived derived;
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DerivedWithVirtual dv;
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llvm::PointerUnion<Z *, float *> z_float(&f);
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llvm::PointerUnion<Z *, float *> raw_z_float(nullptr);
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llvm::PointerUnion<Z *, float *> null_float(static_cast<float *>(nullptr));
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llvm::PointerUnion<long long *, int *, float *> long_int_float(&a);
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llvm::PointerUnion<Z *> z_only(&z);
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llvm::PointerIntPair<llvm::PointerUnion<Z *, float *>, 1> union_int_pair(
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z_float, 1);
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puts("Break here");
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z_float = &derived;
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puts("Break here");
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llvm::PointerUnion<HasVirtual *, float *> virtual_float(&dv);
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puts("Break here");
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// Function-local types stress template_argument lookup in debuggers.
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struct alignas(8) Local {};
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Local local;
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llvm::PointerUnion<Local *, float *> local_float(&local);
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puts("Break here");
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// Variable-width tag encoding: formatter should fall back to void*.
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Align4<0> a4_0;
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Align8<0> a8_0;
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llvm::PointerUnion<Align4<0> *, Align4<1> *, Align4<2> *, Align8<0> *,
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Align8<1> *>
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varwidth(&a4_0);
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llvm::PointerUnion<Align4<0> *, Align4<1> *, Align4<2> *, Align8<0> *,
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Align8<1> *>
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varwidth_tier1(&a8_0);
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puts("Break here");
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}
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