[Bug rtl-optimization/31485] C complex numbers, amd64 SSE, missed optimization opportunity

[bisqwit at iki dot fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=31485

--- Comment #23 from Joel Yliluoma <bisqwit at iki dot fi> ---
(In reply to Jakub Jelinek from comment #21)
> (In reply to Joel Yliluoma from comment #20)
> > Which exceptions would be generated by data in an unused portion of a
> > register?
> 
> addps adds 4 float elements, there is no "unused" portion.
> If some of the elements contain garbage, it can trigger for e.g. the addition
> FE_INVALID, FE_OVERFLOW, FE_UNDERFLOW or FE_INEXACT (FE_DIVBYZERO obviously
> isn't relevant to addition).
> Please read the standard about floating point exceptions, fenv.h etc.

There is “unused” portion, for the purposes of the data use. Same as with
padding in structs; the memory is unused because no part in program relies on
its contents, even though the CPU may load those portions in registers when
e.g. moving and copying the struct. The CPU won’t know whether it’s used or
not.

You mention FE_INVALID etc., but those are concepts within the C standard
library, not in the hardware. The C standard library will not make judgments on
the upper portions of the register. So if you have two float[2]s, and you add
them together into another float[2], and the compiler uses addps to achieve
this task, what is the mechanism that would supposedly generate an exception,
when no part in the software depends and makes judgments on the irrelevant
parts of the register?