Jan Katzer (Aug 15 2024 at 17:16): Jan Katzer (Aug 15 2024 at 17:16):Hi guys,
I'm writing a little toy brainf*ck compiler but I'm having some trouble with x86 codegen
Since brainf*ck primarily uses a big u8 array for storage, I'm emitting lots of
let old_val = b.ins().load(types::I8, mem_flags, real_ptr, static_offset_accum);
let new_val = b.ins().iadd_imm(old_val, insn_val as i64);
b.ins().store(mem_flags, new_val, real_ptr, static_offset_accum);
unfortunately these seem to get turned into manual loads and stores:
movzx rax,BYTE PTR [rcx+0x8000]
add eax,0x1
mov BYTE PTR [rcx+0x8000],al
I would have expected these to get turned into ADD r/m8, r8, i.e. something like
add BYTE PTR [rcx+0x8000], 0x1
Does somebody have some insight into this? Seems odd to me that the x86 backend wouldn't clean that up
Chris Fallin (Aug 15 2024 at 17:21):we have lowering rules for the 32 and 64 bit cases: https://github.com/bytecodealliance/wasmtime/blob/c7756bd2654fe1ac795f9dd9d13de41a31cafbac/cranelift/codegen/src/isa/x64/lower.isle#L3154-L3278
Chris Fallin (Aug 15 2024 at 17:21):it seems someone just needs to add this for the narrower cases; no reason why not other than "someone needs to do it" :-)
Jan Katzer (Aug 15 2024 at 17:35):Ah, thanks for the pointer :D
Seems like a good opportunity to learn how ISLE works
Jan Katzer (Aug 15 2024 at 19:52):This seems to be a bit of a bigger challenge than I hoped, since mem/imm instructions are not supported AT ALL by the x86 backend AFAICT
Chris Fallin (Aug 15 2024 at 19:57):Ah, yes, we've added encodings as we've needed them, so that's a bit of work to fill that out
Jan Katzer (Aug 15 2024 at 19:58):No worries, makes me do something and actually learn how it works :D
Jan Katzer (Aug 15 2024 at 19:59):Quick question, what would be the best type for immediates in ISLE? I've seen i32 be used as a "general type" but I can't figure out how to get one from an iconst or whatever the second operand should be
Chris Fallin (Aug 15 2024 at 20:01):we've tended to use u64s and should have an extractor that converts from iconst's parameter (u64_from_imm64 or somesuch? can't look at the moment)
Last updated: Dec 23 2024 at 12:05 UTC