x86_64 ABI: pass structs indirectly if there are no more available registers #11344
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straight-shoota
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Fixes #9519
Unix x86_64 ABI defines CPU registers
rdi,rsi,rdx,rcx,r8,r9(for integer values) andxmm0throughxmm7(for floating point values) for passing arguments to functions. Small structures (smaller than 64 bytes, or 8 eight-bytes) can be passed using registers as well, by passing individual fields into separate registers. But if the number of arguments is larger than the available registers, the remaining values are passed through the stack. LLVM manages this for scalar values, but for aggregates it needs thebyvalattribute to the argument type added.This patch is heavily inspired by the Rust implementation (from where the original ABI code was taken). The previously added spec by #9520 is enabled and I also added a new one to cover the
StructRetscenario which effectively introduces an extra hidden function argument as a pointer to the caller-allocated return value.