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POIn GCC-style extended inline asm, is it possible to output a "virtualized" boolean value, e.g. the carry flag?
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copied!<p>If I have the following C++ code to compare two 128-bit unsigned integers, with inline amd-64 asm:</p> <pre><code>struct uint128_t { uint64_t lo, hi; }; inline bool operator< (const uint128_t &a, const uint128_t &b) { uint64_t temp; bool result; __asm__( "cmpq %3, %2;" "sbbq %4, %1;" "setc %0;" : // outputs: /*0*/"=r,1,2"(result), /*1*/"=r,r,r"(temp) : // inputs: /*2*/"r,r,r"(a.lo), /*3*/"emr,emr,emr"(b.lo), /*4*/"emr,emr,emr"(b.hi), "1"(a.hi)); return result; } </code></pre> <p>Then it will be inlined quite efficiently, but with one flaw. The return value is done through the "interface" of a general register with a value of 0 or 1. This adds two or three unnecessary extra instructions and detracts from a compare operation that would otherwise be fully optimized. The generated code will look something like this:</p> <pre><code> mov r10, [r14] mov r11, [r14+8] cmp r10, [r15] sbb r11, [r15+8] setc al movzx eax, al test eax, eax jnz is_lessthan </code></pre> <p>If I use "sbb %0,%0" with an "int" return value instead of "setc %0" with a "bool" return value, there's still two extra instructions:</p> <pre><code> mov r10, [r14] mov r11, [r14+8] cmp r10, [r15] sbb r11, [r15+8] sbb eax, eax test eax, eax jnz is_lessthan </code></pre> <p>What I want is this:</p> <pre><code> mov r10, [r14] mov r11, [r14+8] cmp r10, [r15] sbb r11, [r15+8] jc is_lessthan </code></pre> <p>GCC extended inline asm is wonderful, otherwise. But I want it to be just as good as an intrinsic function would be, in every way. I want to be able to directly return a boolean value in the form of the state of a CPU flag or flags, without having to "render" it into a general register.</p> <p>Is this possible, or would GCC (and the Intel C++ compiler, which also allows this form of inline asm to be used) have to be modified or even refactored to make it possible?</p> <p>Also, while I'm at it — is there any other way my formulation of the compare operator could be improved?</p>

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