ActOnPragmaFloatControl() restores last fp eval method from a different location than where it was saved

[mrwalker61]

See clang/lib/Sema/SemaAttr.cpp.

Here PP.setLastFPEvalMethod(PP.getCurrentFPEvalMethod()) saves the eval method (case PFC_NoPrecise).

PP.setCurrentFPEvalMethod(SourceLocation(), CurFPFeatures.getFPEvalMethod()) restores the eval method (case PFC_Pop).

Should it not be restored like PP.setCurrentFPEvalMethod(SourceLocation(), PP.getLastFPEvalMethod())?

Compiling a C++ snippet that did #include <numeric> (from MSVC 14, as distributed with Microsoft Visual Studio 2022) with a newly built Clang 15 produced an outpour of warnings (32-bit x86 build without SSE enabled). This was caused by the initial value 2 for the fp eval method suddenly changing to 0, an illegal value on the platform. Debugging the issue I can see the value 2 is not restored in this code, in effect just set back to a default (0), but this is not even the correct default on x86 (which is 2).

[mrwalker61]

Note that 0 is the correct default also on x86 WITH SSE, otherwise it is 2.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[mrwalker61]

Minimal testcase (compile for 32-bit x86 without SSE):

#pragma float_control( precise, off, push )
#pragma float_control( pop )

int main() {
    return 0;
}

[mrwalker61]

However it seems to me push and pop does not work properly with #pragma float_control on any architecture. See this example (referred to below as test.cpp) with more extensive test output:

#include <stdio.h>
#include <cfloat>

int val0 = FLT_EVAL_METHOD;
#pragma float_control( precise, off, push )
int val1 = FLT_EVAL_METHOD;
#pragma float_control( precise, on, push )
int val2 = FLT_EVAL_METHOD;
#pragma float_control( pop )
int val3 = FLT_EVAL_METHOD;
#pragma float_control( pop )
int val4 = FLT_EVAL_METHOD;

int main() {
    printf("FLT_EVAL_METHOD=%2d\n", val0);
    printf("FLT_EVAL_METHOD=%2d\n", val1);
    printf("FLT_EVAL_METHOD=%2d\n", val2);
    printf("FLT_EVAL_METHOD=%2d\n", val3);
    printf("FLT_EVAL_METHOD=%2d\n", val4);
    return 0;
}

[mrwalker61]

Running "clang-cl.exe -m32 test.cpp && test.exe" results in:

FLT_EVAL_METHOD= 0
FLT_EVAL_METHOD=-1
FLT_EVAL_METHOD= 0
FLT_EVAL_METHOD= 0
FLT_EVAL_METHOD= 0

Not exactly what I expected at least.

[mrwalker61]

But with "clang-cl.exe /arch:IA32 -m32 test.cpp && test.exe" it really becomes weird, and this was my original issue:

FLT_EVAL_METHOD= 2
FLT_EVAL_METHOD=-1
FLT_EVAL_METHOD= 2
FLT_EVAL_METHOD= 2
FLT_EVAL_METHOD= 0

[mrwalker61]

Fixed by llvm/llvm-project-release-prs#360