You wrote: >在 2023-05-26 14:46, Stefan Kanthak 写道: >> OOPS: why does GCC (ab)use the SSE2 alias "Willamette New Instruction Set" >> (... ...) >> OUCH: why does it FAIL to REALLY use SSE2, as shown in the comments on the >> right side? > > Please stop yelling like that. It makes you look like a naughty pupil.
That's EMPHASISING, kid! >> 14 instructions in 33 bytes # 11 instructions in 32 bytes >> >> OUCH: why does GCC abuse EBX (and ECX too) and performs a superfluous >> memory write? > > Apart from the SSE question: You are performing 64-bit arithmetic on a 32-bit > machine, which GCC isn't good at. So it's REALLY time to fix that. > The preferred way to check whether a 64-bit integer is a power of two is to > cast it > to a float, then examine whether its 23-bit mantissa is all zeroes: Feel free to propose this alternative here (better elsewhere, where you'll earn less laughter). But don't forget that this 23-bit mantissa will be all zeroes for quite some 64-bit (and even 32-bit) integers which are no power of 2, for example 0x8000003fffffffff, and that both FILD and CVT2SI2SS only work on SIGNED integers. I instead prefer to show that and how GCCs current code generator fails to optimise properly. Stefan
