On 05/04/2015 12:33 PM, Richard Henderson wrote: > > (0) The C level output variable should be an integral type, from bool on up. > > The flags are a scarse resource, easily clobbered. We cannot allow user code > to keep data in the flags. While x86 does have lahf/sahf, they don't exactly > perform well. And other targets like arm don't even have that bad option. > > Therefore, the language level semantics are that the output is a boolean store > into the variable with a condition specified by a magic constraint. > > That said, just like the compiler should be able to optimize > > void bar(int y) > { > int x = (y <= 0); > if (x) foo(); > } > > such that we only use a single compare against y, the expectation is that > within a similarly constrained context the compiler will not require two tests > for these boolean outputs. > > Therefore: > > (1) Each target defines a set of constraint strings, > > E.g. for x86, wherein we're almost out of constraint letters, > > ja aux carry flag > jc carry flag > jo overflow flag > jp parity flag > js sign flag > jz zero flag >
I would argue that for x86 what you actually want is to model the *conditions* that are available on the flags, not the flags themselves. There are 16 such conditions, 8 if we discard the inversions. It is notable that the auxiliary carry flag has no Jcc/SETcc/CMOVcc instructions; it is only ever consumed by the DAA/DAS instructions which makes it pointless to try to model it in a compiler any more than, say, IF. > (2) A new target hook post-processes the asm_insn, looking for the > new constraint strings. The hook expands the condition prescribed > by the string, adjusting the asm_insn as required. > > E.g. > > bool x, y, z; > asm ("xyzzy" : "=jc"(x), "=jp"(y), "=jo"(z) : : ); Other than that, this is exactly what would be wonderful to see. -hpa