Question

Sorry for so many questions, but I’ve encountered yet another cryptic error trying to compile the following inline assembly (with -fasm-blocks) which works in MSVC, but apparently not in GCC and wasn’t able to deal with it:

unsigned char testData = 128;

__asm
{
    // ...
    mov al, testData
    mov ah, al // error: asm-specifier for variable '%al' conflicts with asm clobber list
    shl eax, 16
    // ...
};

What is this clobber list and what is wrong with it?

I also tried to change optimization level, but it had no effect.

Answer 1

This has to be some bug in gcc (maybe __asm blocks have implicit clobbering). Anyway there are many workarounds:

__asm
{
    // ...
    mov ah, testData
    mov al, ah
    shl eax, 16
    // ...
};

or

__asm
{
    // ...
    mov al, testData
    mov ah, testData
    shl eax, 16
    // ...
};

or

__asm
{
    // ...
    movzx eax, testData
    imul eax, 0x0101
    shl eax, 16
    // ...
};