3 ms·

My first thought was "now what is the calling convention for float parameters again? they are passed in registers right? the compiler can probably arrange so th

by im3w1l 4y ago

My first thought was "now what is the calling convention for float parameters again? they are passed in registers right? the compiler can probably arrange so they don't have to actually be copied" and then I realized it will probably even inline it. Anyway, assuming it's not inlined I would guess pass-by-copy, maybe with an occasional exception in code with heavy register pressure. Edit: Actually since it's a structure, the calling convention is to memory allocate it and pass a pointer, doh. So it should actually compile the same.

unsafecast 4y ago
> Edit: Actually since it's a structure, the calling convention is to memory allocate it and pass a pointer, doh. So it should actually compile the same. Depending on calling convention, the structure may be spread out into registers.
masklinn 4y ago
> Edit: Actually since it's a structure, the calling convention is to memory allocate it and pass a pointer, doh. So it should actually compile the same. FWIW the AMD64 SysV v1.0 psABI allows structures of up to 8 members to be passed via registers. Though older revisions limit that to 2 (and it's unclear whether MS's divergent ABI allows aggregates to be splat at all. As sad as it's unsurprising, it does not look like LLVM (linux?) has followed up, on godbolt a 2-struct passes everything via registers but a 3-struct passes everything via the stack. Maybe there's a magic flag to use the 1.0 ABI, but a quick googling didn't reveal one. ICC doesn't seem to have followed up either.
- im3w1l 4y ago
  How can an abi allow but not require passing them via registers? Isn't the point of a convention that everyone does it the same way? Other wise the caller could put it on the stack and the callee look for it in a register? Actually reading the word abi made a little neuron light up. To what extent does rust even follow that abi?

by im3w1l 4y ago

unsafecast 4y ago

> Edit: Actually since it's a structure, the calling convention is to memory allocate it and pass a pointer, doh. So it should actually compile the same. Depending on calling convention, the structure may be spread out into registers.

masklinn 4y ago

> Edit: Actually since it's a structure, the calling convention is to memory allocate it and pass a pointer, doh. So it should actually compile the same. FWIW the AMD64 SysV v1.0 psABI allows structures of up to 8 members to be passed via registers. Though older revisions limit that to 2 (and it's unclear whether MS's divergent ABI allows aggregates to be splat at all. As sad as it's unsurprising, it does not look like LLVM (linux?) has followed up, on godbolt a 2-struct passes everything via registers but a 3-struct passes everything via the stack. Maybe there's a magic flag to use the 1.0 ABI, but a quick googling didn't reveal one. ICC doesn't seem to have followed up either.

im3w1l 4y ago
How can an abi allow but not require passing them via registers? Isn't the point of a convention that everyone does it the same way? Other wise the caller could put it on the stack and the callee look for it in a register? Actually reading the word abi made a little neuron light up. To what extent does rust even follow that abi?