On Sunday, 18 July 2021 at 11:42:24 UTC, Guillaume Piolat wrote:
Is anyone versed in LLVM inline asm?
I know how to generate SIMD unary op with:
return __asm!int4("pmovsxwd $1,$0","=x,x",a);
but I struggle to generate 2-operands SIMD ops like:
return __asm!int4("paddd $1,$0","=x,x",a, b);
If you know how to do it => https://d.godbolt.org/z/ccM38bfMT
it would probably help build speed of SIMD heavy code, also -O0
performance
Also generating the right instruction is good but it must
resist optimization too, so proper LLVM constraints is needed.
It would be really helpful if someone has understood the
cryptic rules of LLVM assembly constraints.
Yeah I can confirm it's aweful. Took me hours to understand how
to use it a bit (my PL has [an
interface](https://styx-lang.gitlab.io/styx/primary_expressions.html#asmexpression) for LLVM asm)
You need to add a "x" to the constraint string
return __asm!int4("paddd $1,$0","=x,x,x",a, b);
- **=x** says "returns in whatever is has to"
- **x** (1) is the constraint for input `a`, which is passed as
operand **$0**
- **x** (2) is the constraint for input `b`, which is passed as
operand **$1**
So the thing to get is that the output constraint does not
consume anything else, it is standalone.
