On Mon, Jun 30, 2025 at 7:05 PM Alexander Monakov <amona...@ispras.ru> wrote: > > > On Mon, 30 Jun 2025, Richard Biener wrote: > > > > * gcc.target/i386/pr116979.c > > > > > > Here we check that FMA is used for complex multiplication. This is a case > > > where use of FMA is detrimental for accuracy, but I guess we can choose > > > to preserve the status quo. I can look into teaching > > > expand_complex_multiplication to emit FMAs. > > > > Yeah, I guess that's within -ffp-contract=on constraints. When you say it's > > detrimental for accuracy, is that a general statement for complex multiply > > lowering? If so we possibly do not want FMAs? > > We may want FMAs when completely disregarding accuracy for speed > (-ffast-math), > but otherwise, yes, I meant that in general our strategy is not good. We are > lowering complex multiplication by components > > (x + yi)*(z + wi) = (x*z - y*w) + (x*w + y*z)i > > and when FMA is available, we end up with something like > > fma(x, z, -y*w) > > for the real part and likewise fma(x, w, y*z) for the imaginary part, which is > asymmetrical with respect to operands, and, for instance, yields wrong results > for conjugate vectors: take z=x and w=-y, then imaginary part fma(x, w, y*z) > is > fma(x, -y, y*x), which is negated round-off tail of y*x, but should be zero. > > FMA is useful for improving accuracy of sum/difference of two products, where > you'd compute x*w + y*z as > > xw = x*w > xw_tail = fma(x, w, -xw) > tmp = fma(y, z, xw) > return tmp + xw_tail > > but apart from this, I'd say we don't want to emit FMAs in > expand_complex_multiplication. LLVM seems to produce FMAs for complex product > only under -ffp-contract=fast. > > What are the next steps here, then?
For this case I would then suggest to not change complex lowering to emit FMAs and adjust the testcases to pass -ffp-contract=fast instead, possibly even add a testcase variant with -ffp-contract=on checking FMAs are not used with a rationale present in a comment. Richard. > > Thank you. > Alexander
