[spirv] Fix firstbithigh/low signedness #5665
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The SPIR-V backend was incorrectly choosing to emit either FindSMsb or FindUMsb based on whether the result type of the call expression was signed or unsigned. Since the AST result type was always unsigned (and wrapped in a cast if necessary), this meant FindSMsb was never generated. FindSMsB should however be generated when the argument type is signed. This is indicated by the HLSL intrinsic op code (firstbithigh for signed and ufirstbithigh for unsigned). The SPIR-V generated now matches the DXIL generated. This bug was discovered while investigating microsoft#4702, so at the same time some error checking is added to make unimplemented cases explicit. Because the underlying GLSL.std.450 instructions FindSMsb, FindUMsb and FindILsB are currently limited to 32-bit width components, we now emit an error messages when this would otherwise result in invalid SPIR-V.
s-perron
approved these changes
Sep 7, 2023
cassiebeckley
approved these changes
Sep 7, 2023
ShchchowAMD
pushed a commit
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that referenced
this pull request
Sep 11, 2023
The SPIR-V backend was incorrectly choosing to emit either FindSMsb or FindUMsb based on whether the result type of the call expression was signed or unsigned. Since the AST result type was always unsigned (and wrapped in a cast if necessary), this meant FindSMsb was never generated. FindSMsB should however be generated when the argument type is signed. This is indicated by the HLSL intrinsic op code (firstbithigh for signed and ufirstbithigh for unsigned). The SPIR-V generated now matches the DXIL generated. This bug was discovered while investigating microsoft#4702, so at the same time some error checking is added to make unimplemented cases explicit. Because the underlying GLSL.std.450 instructions FindSMsb, FindUMsb and FindILsB are currently limited to 32-bit width components, we now emit an error messages when this would otherwise result in invalid SPIR-V.
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The SPIR-V backend was incorrectly choosing to emit either FindSMsb or FindUMsb based on whether the result type of the call expression was signed or unsigned. Since the AST result type was always unsigned (and wrapped in a cast if necessary), this meant FindSMsb was never generated. FindSMsB should however be generated when the argument type is signed. This is indicated by the HLSL intrinsic op code (firstbithigh for signed and ufirstbithigh for unsigned). The SPIR-V generated now matches the DXIL generated.
This bug was discovered while investigating #4702, so at the same time some error checking is added to make unimplemented cases explicit. Because the underlying GLSL.std.450 instructions FindSMsb, FindUMsb and FindILsB are currently limited to 32-bit width components, we now emit an error messages when this would otherwise result in invalid SPIR-V.