Issue with __builtin_remainder expansion on i386

[FX <fxcoudert at gmail dot com>]

Hi all,

I have just submitted a patch emitting some new floating-point code from the Fortran front-end, improving our IEEE support there: https://gcc.gnu.org/ml/gcc-patches/2014-09/msg02444.html

However, in one of the cases where we emit a call to __builtin_remainderf(), we get wrong code generation on i386. It is peculiar because:

 - the wrong code occurs at all optimization levels, and the following flags (any or all) do not affect it: -mfpmath=sse -msse2 -fno-unsafe-math-optimizations -frounding-math -fsignaling-nans
 - the wrong code does not occur with -ffloat-store
 - the code generate looks fine by every aspect I could try. I could not generate a direct C testcase, unfortunately, but it is equivalent to:

          D.2303 = __builtin_remainderf (ieee_value_4 (&C.2299, &D.2301), 1.0e+0);

where ieee_value_4() is a function that returns a positive infinity (for the specific values of arguments passed). However, if I split it in:

          x = ieee_value_4 (&C.2289, &D.2293);
          D.2297 = __builtin_remainderf (x, 1.0e+0);

then the wrong result does not occur anymore either. Whatâs bugging me is that, with and without -ffloat-store, the same dump trees are generated. So I donât know how to get further on debugging this. I do think itâs something wrong with the expansion of __builtin_remainderf(), especially as it depends on -ffloat-store.


I attach the source code triggering it (built on trunk with my patch at https://gcc.gnu.org/ml/gcc-patches/2014-09/msg02444.html), the tree dumps, and the assembler outputs (a.s without -ffloat-store, and a.s.floatstore with it). I can clearly see, from the assembly diff, that something is indeed changing but I cannot understand what.

Thanks in advance if you have any hint to share, or get a minute to look at the dumps/assembly.

FX

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