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libzigc: Implement tanhf, tanh, modfl, modff, modf, acoshf, asin #31536

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andrewrk merged 10 commits from mihael/zig:libzigc/more-math-functions into master 2026年03月18日 06:34:22 +01:00
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Most of these implementations were just straightforward calling of the Zig standard library, expect for the modf libc functions.
The changes were tested at each commit, and the results are attached to the commit messages.

libc's modf behaves a bit differently from Zig's, so I had to write a little more to cover the edge cases and add tests for them. I've then generalized the solution so it could be reused for modff and modfl as well.

Contributes to #30978

Most of these implementations were just straightforward calling of the Zig standard library, expect for the `modf` libc functions. The changes were tested at each commit, and the results are attached to the commit messages. libc's `modf` behaves a bit differently from Zig's, so I had [to write a little more](https://codeberg.org/mihael/zig/commit/cc1217543d3d5f76105e71bd56a67921dafdd0c6) to cover the edge cases and add tests for them. I've then generalized the solution so it could be reused for [`modff`](https://codeberg.org/mihael/zig/commit/5ff144bd97078cdadf2460b44a05f549ef10f411) and [`modfl`](https://codeberg.org/mihael/zig/commit/5f57567d1f2524142a8478acb7b3eb7dbba79101) as well. Contributes to #30978
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andrewrk requested changes 2026年03月17日 00:09:38 +01:00
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Thanks for working on this. One issue-

Thanks for working on this. One issue-
lib/c/math.zig Outdated
@ -155,0 +182,4 @@
return0.0;
}
// Avoids raising the INVALID flag on qemu-riscv
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zig's libc implementation is not an appropriate place for qemu workarounds. If there's a bug in qemu it needs to be fixed upstream in the qemu project, and not worked around here.

zig's libc implementation is not an appropriate place for qemu workarounds. If there's a bug in qemu it needs to be fixed upstream in the qemu project, and not worked around here.
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It's clumsily worded. I wanted to signal I came across this issue while using Qemu, not an actual RISC-V board, but it comes across as a Qemu bug workaround. But, on second thought, that's hardly relevant, so I ought to remove the comment altogether.

Without this block, libc-test suite will fail for modf and modff implementations on RISC-V by raising the INVALID exception when NaN is passed as the argument, but pass for modfl.

I've been investigating a bit and I'm unsure if it's a bug in Qemu or just expected RISC-V behavior. There are instructions that can easily trigger that exception, such as:

that is, they set the invalid operation exception flag if either input is NaN.

Without the early return, there are bound to be some arithmetic operations involving the NaN (trunc, subtraction at the very least) and thus a chance to raise the INVALID exception / get a signaling NaN. I'd have to drop a little lower and verify what it gets compiled into, and see if any of those instructions would cause this as per RISC-V docs (expected behavior) or if it's some kind of Qemu bug.

If anyone knows more, I'd love for them to chip in because I'm truly no RISC-V expert. 😄 I'd be interested in running this on actual RISC-V hardware, but sadly I don't have it.

All that being said, I still like having an early return on invalid input like that, so I'd leave the NaN check regardless -- unless there's a reason why we might prefer not having it. It also helping out with this issue when running tests is a happy little accident.

It's clumsily worded. I wanted to signal I came across this issue while using Qemu, not an actual RISC-V board, but it comes across as a Qemu bug workaround. But, on second thought, that's hardly relevant, so I ought to remove the comment altogether. Without this block, `libc-test` suite will fail for `modf` and `modff` implementations on RISC-V by raising the `INVALID` exception when `NaN` is passed as the argument, but pass for `modfl`. I've been investigating a bit and I'm unsure if it's a bug in Qemu or just expected RISC-V behavior. There are instructions that can easily trigger that exception, such as: - [Floating-Point Comparison](https://docs.riscv.org/reference/isa/unpriv/f-st-ext.html#20-1-8-single-precision-floating-point-compare-instructions) >that is, they set the invalid operation exception flag if either input is `NaN`. Without the early return, there are bound to be some arithmetic operations involving the `NaN` (trunc, subtraction at the very least) and thus a chance to raise the `INVALID` exception / get a signaling `NaN`. I'd have to drop a little lower and verify what it gets compiled into, and see if any of those instructions would cause this as per RISC-V docs (expected behavior) or if it's some kind of Qemu bug. If anyone knows more, I'd love for them to chip in because I'm truly no RISC-V expert. 😄 I'd be interested in running this on actual RISC-V hardware, but sadly I don't have it. All that being said, I still like having an early return on invalid input like that, so I'd leave the `NaN` check regardless -- unless there's a reason why we might prefer not having it. It also helping out with this issue when running tests is a happy little accident.
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I see, thanks for explaining.

I'd be interested in running this on actual RISC-V hardware, but sadly I don't have it.

Good news! Thanks to @alexrp, the CI jobs with "riscv" in the name are running on actual RISC-V hardware. It's relatively slow however, so we run those checks only on master branch commits. qemu is good enough to land a PR, and we can follow-up if any problems are discovered by those checks.

I see, thanks for explaining. > I'd be interested in running this on actual RISC-V hardware, but sadly I don't have it. Good news! Thanks to @alexrp, the CI jobs with "riscv" in the name are running on actual RISC-V hardware. It's relatively slow however, so we run those checks only on master branch commits. qemu is good enough to land a PR, and we can follow-up if any problems are discovered by those checks.
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