support fesetround(FE_UPWARD)?

[Innovgame]

Please include the following in your bug report:

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.43 (a6b8143)
clang version 17.0.0 (https://github.com/llvm/llvm-project 71513a71cdf380efd6a44be6939e2cb979a62407)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: D:\emsdk\upstream\bin

Failing command line in full:
fesetround(FE_UPWARD) not work. My code relies on it, what should I do?

[renqh123]

I have also encountered this issue recently and I really hope to find an effective solution.

[sbc100]

Can you give an example that fails? Does it fail to build or does it fail to produce the correct results?

[sbc100]

IIUC this is a feature is missing from the underlying WebAssembly instruction set. IIUC we have no way to control the floating point unit in this way. @tlively can you confirm?

Update: It looks like there is an active proposal to add rounding mode control: https://github.com/WebAssembly/rounding-mode-control. The discussion that let to this proposal is WebAssembly/design#1456. However from the looks of the proposal it doesn't look like it would provide a way to implement fesetround which requires the ability to change the ambient rounding mode. Is my understanding correct @tlively ?

[tlively]

Yes, that is correct. While having an ambient rounding mode was considered, most folks want per-operation rounding mode instead because it is more in line with WebAssembly's design principles. The people in that discussion might be interested in hearing more about your use case, @renqh123. For the time being, you will have to change your code to not depend on an ambient rounding mode.