Intel 风格内联汇编和 AT&T 风格、C++ 中的变量引用

Question

I need to compile some assembly code in both Visual Studio and an IDE using G++ 4.6.1. The -masm=intel flag works as long as I do not reference and address any variables, which however I need to do.

I considered using intrinsics, but the compiled assembly is not optimal at all (for instance I cannot define the sse-register to be used and thus no pipe optimization is possible).

Consider these parts of code (inte style assembly):

mov       ecx, dword ptr [p_pXcoords]
mov       edx, dword ptr [p_pYcoords]
movhpd    xmm6, qword ptr [oAvgX]
movhpd    xmm7, qword ptr [oAvgY]
movlpd    xmm6, qword ptr [oAvgX]
movlpd    xmm7, qword ptr [oAvgY]

where p_pXcoords and p_pYcoords are doublde* arrays and function parameters, oAvgX and oAvgY simpled double values.

Another line of code is this, which is in the middle of an assembly block:

movhpd    xmm6, qword ptr [oAvgY]

in other words, I need to access variables and use them within specific sse registers in the middle of the code. How can I do this with AT & T syntax, best: can I do this with a g++ compiler using the -masm flag?

Is there any way at all using one assembly code for both VS and a g++ 4.6.1 based compiler

Answer 1

You can certainly tell GCC which SSE register to use for each variable:

register __m128i x asm("xmm6");

But I guess VS does not support that. (I am also a little surprised you need it for decent performance. Register assignment and instruction scheduling are two of the most basic things an optimizing compiler knows. You sure you enabled optimization :-) ?)

I would probably just write two functions, one using intrinsics, and one using asm for whichever compiler does not know how to schedule instructions properly.