# Issues with asm syntax

**URL:** <https://ziggit.dev/t/issues-with-asm-syntax/17903>\
**Category:** Help\
**Tags:** language, compiler\
**Created:** [October 9, 2026, 12:47pm UTC](https://ziggit.dev/t/issues-with-asm-syntax/17903 "2026-10-09T12:47:37Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![lassade](https://ziggit.dev/user_avatar/ziggit.dev/lassade/32/569_2.png) [@lassade](https://ziggit.dev/u/lassade)\
**Post date:** [October 9, 2026, 12:47pm UTC](https://ziggit.dev/t/issues-with-asm-syntax/17903/1 "2026-10-09T12:47:37Z")

</div>

I have 2 problems:

**1. How do tie output input registers?**

I have a optimized dot function for sse4.1, but it manually allocates registers, thats less than ideal and will spawn a ton of `vmovaps` to copy from one register to another, LLVM has a syntax for it [LLVM Language Reference Manual — LLVM 10 documentation](https://releases.llvm.org/10.0.0/docs/LangRef.html#input-constraints) that looks like `=r,0` but zig can’t compile with that

```zig
pub inline fn dot_splat_sse41(v0: F32x4, v1: F32x4) F32x4 {
  return asm (
    \\dpps $0xff, %[v1], %[v0]
    : [ret] "={xmm0}" (-> F32x4), // output
    : [v0] "{xmm0}" (v0), // inputs
      [v1] "x" (v1));
}

```

**2. Multiple outputs and input constantness**

The same link above states:

> … It is not permitted for the asm to write to any input register or memory location (unless that input is tied to an output).

Now I implemented an optimized version of `@reduce(.Min, v);` for sse2, here I use the input v as a temp register, zig inline asm doesn’t allow multiple outputs, so what now?

Even if a choose to use a different register, I don’t know how do allocate a temp register.

```zig
pub inline fn f32x4_reduce_min(v: F32x4) f32 {
  if (sse2) {
    const out = asm (
      \\movaps %[v], %[r]
      \\unpckhpd %[r], %[v]
      \\minps %[v], %[r]
      \\pshufd $0x55, %[r], %[v]
      \\minps %[v], %[r]
      : [r] "=&x" (-> F32x4), // output
      : [v] "x" (v) // todo: LLVM prohibits from writing to %[v] because he isn't tied to an output
      : .{}
    );
return out[3];
  } else {
    @reduce(.Min, v); // todo: NEON instruction for this
  }
}

```

Edit: fixed `f32x4_reduce_min` output type

---

<div class="post-metadata">

**Author:** ![lassade](https://ziggit.dev/user_avatar/ziggit.dev/lassade/32/569_2.png) [@lassade](https://ziggit.dev/u/lassade)\
**Post date:** [October 9, 2026, 1:06pm UTC](https://ziggit.dev/t/issues-with-asm-syntax/17903/2 "2026-10-09T13:06:43Z")

</div>

I figureout 1, it turns out the link is talking about the LLMV IR syntax, gnu documentaion [Simple Constraints (Using the GNU Compiler Collection (GCC))](https://gcc.gnu.org/onlinedocs/gcc/Simple-Constraints.html#Simple-Constraints) doesn’t have a `,0` constraint only numbers that must be passed to the input like so:

```zig
pub inline fn dot_splat_sse41(v0: F32x4, v1: F32x4) F32x4 {
  return asm (
    \\dpps $0xff, %[v1], %[v0]
    : [ret] "=x" (-> F32x4), // output
    : [v0] "0" (v0),
      [v1] "x" (v1));
}

```

---

<div class="post-metadata">

**Author:** ![gwenzek](https://ziggit.dev/user_avatar/ziggit.dev/gwenzek/32/8137_2.png) [@gwenzek](https://ziggit.dev/u/gwenzek)\
**Post date:** [October 9, 2026, 4:06pm UTC](https://ziggit.dev/t/issues-with-asm-syntax/17903/3 "2026-10-09T16:06:35Z")

</div>

Best i can suggest for 2 is to use an harcoded register for the second output. Explicitly clobber it in the main asm block, then use a second to copy out from the harcoded register.

In general forcing specific register may harm register allocation, but if you have a low density of such function it’s likely that LLVM can still generate optimal assembly