> Traps don't quite work the way you'd want in C [1]; there's no way to catch a trap that occurs in a specific region of code.

Ah, unless you're in NT land, which makes lexical scoping of traps very easy: https://github.com/tpn/tracer/blob/0224d94b8d17fe74c39cec285...

        //
        // Verify the guard page is working properly by wrapping an attempt to
        // write to it in a structured exception handler that will catch the
        // access violation trap.
        //
        // N.B. We only do this if we're not actively being debugged, as the
        //      traps get dispatched to the debugger engine first as part of
        //      the "first-pass" handling logic of the kernel.
        //

        if (!IsDebuggerPresent()) {

            CaughtException = FALSE;

            TRY_PROBE_MEMORY{

                *Unusable = 1;

            } CATCH_EXCEPTION_ACCESS_VIOLATION{

                CaughtException = TRUE;

            }

            ASSERT(CaughtException);
        }

The helper #defines: https://github.com/tpn/tracer/blob/0224d94b8d17fe74c39cec285..., e.g.

    #define TRY_TSX __try
    #define TRY_AVX __try
    #define TRY_AVX512 __try
    #define TRY_AVX_ALIGNED __try
    #define TRY_AVX_UNALIGNED __try
    
    #define TRY_SSE42 __try
    #define TRY_SSE42_ALIGNED __try
    #define TRY_SSE42_UNALIGNED __try
    
    #define TRY_PROBE_MEMORY __try
    #define TRY_MAPPED_MEMORY_OP __try
    
    #define CATCH_EXCEPTION_ILLEGAL_INSTRUCTION __except(     \
        GetExceptionCode() == EXCEPTION_ILLEGAL_INSTRUCTION ? \
            EXCEPTION_EXECUTE_HANDLER :                       \
            EXCEPTION_CONTINUE_SEARCH                         \
        )
    
    #define CATCH_EXCEPTION_ACCESS_VIOLATION __except(     \
        GetExceptionCode() == EXCEPTION_ACCESS_VIOLATION ? \
            EXCEPTION_EXECUTE_HANDLER :                    \
            EXCEPTION_CONTINUE_SEARCH                      \
        )
    
    #define CATCH_STATUS_IN_PAGE_ERROR __except(     \
        GetExceptionCode() == STATUS_IN_PAGE_ERROR ? \
            EXCEPTION_EXECUTE_HANDLER :              \
            EXCEPTION_CONTINUE_SEARCH                \
        )
    
    #define CATCH_STATUS_IN_PAGE_ERROR_OR_ACCESS_VIOLATION __except( \
        GetExceptionCode() == STATUS_IN_PAGE_ERROR ||                \
        GetExceptionCode() == EXCEPTION_ACCESS_VIOLATION ?           \
            EXCEPTION_EXECUTE_HANDLER :                              \
            EXCEPTION_CONTINUE_SEARCH                                \
        )

Also allows you to do fun things like this for testing if you can do an AVX512 op (although this is not the supported way of doing things):

    #pragma optimize("", off)
    static
    NOINLINE
    VOID
    CanWeUseAvx512(PBOOLEAN UseAvx512Pointer)
    {
        BOOLEAN UseAvx512 = TRUE;
        TRY_AVX512 {
            ZMMWORD Test1 = _mm512_set1_epi64(1);
            ZMMWORD Test2 = _mm512_add_epi64(Test1, Test1);
            UNREFERENCED_PARAMETER(Test2);
        } CATCH_EXCEPTION_ILLEGAL_INSTRUCTION{
            UseAvx512 = FALSE;
        }
        *UseAvx512Pointer = UseAvx512;
    }
    #pragma optimize("", on)

https://github.com/tpn/tracer/blob/0224d94b8d17fe74c39cec285...

The structured exception handling protocol used by NT is really quite elegant.