On Tue, Nov 17, 2020 at 9:56 PM Brian Paul <bri...@vmware.com> wrote: > > On 11/17/2020 11:49 AM, Ian Romanick wrote: > > On 11/17/20 9:25 AM, Brian Paul wrote: > >> > >> Using the Intel Vulkan driver, we've found some cases where SpvOpSelect > >> is returning -0.0 (negative zeros) instead of normal 0.0 depending on > >> the arguments. > > > > Do you have a specific test case that fails? > > Yeah, but as with the NMin/NMax issue it's not a simple test case. It > comes from a Windows WHCK test suite. > > > > It seems like on some platforms there was an errata about the version of > > the SEL instruction that is used for min or max that can return the > > wrong signed zero in some cases. > > > > It's also possible that some optimizations are causing problems. I > > don't remember exactly how it works in SPIR-V, but does marking those > > SPIR-V instructions as precise (that's what it was in GLSL) make a > > difference? > > AFAIK, there's only a SPIR-V decoration for tagging things for relaxed > precision.
The SPIR-V equivalent of "precise" is NoContract. But for emulating DX stuff you're supposed to use KHR_shader_float_controls which was specifically designed for emulating DX floating-point requirements. In NIR that just marks everything as "exact" if you force correct NaN/signed zero/Inf handling. > > -Brian > > > > >> I'm wondering if "SpvOpSelect x, a, b" for floats is being implemented > >> with something like "a*x + b*(1-x)" ? That might explain where the > >> negative zeros are coming from. > >> > >> Our work-around is to implement selection with bitwise operations: (a & > >> x) | (b & ~x) > >> > >> It seems to me that SpvOpSelect shouldn't interpret the bits and just > >> return an exact copy of the argument. > >> > >> -Brian > >> _______________________________________________ > >> mesa-dev mailing list > >> mesa-dev@lists.freedesktop.org > >> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.freedesktop.org%2Fmailman%2Flistinfo%2Fmesa-dev&data=04%7C01%7Cbrianp%40vmware.com%7C5e1ffabf0f7c47a2f48308d88b298304%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C1%7C637412357691565798%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=SdzrOYvGGFraVg61OGvIQ6NJrT2i7fye%2Fy03XoZOi7E%3D&reserved=0 > > > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/mesa-dev _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
