[Bug target/114659] gcc miscompiles a __builtin_memcpy on i386, leading to wrong results for SNaN

["bruno at clisp dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659

--- Comment #8 from Bruno Haible <bruno at clisp dot org> ---
(In reply to Andrew Pinski from comment #6)
> I doubt there is not much to be done here.

I see it as an incorrect modelization of the x87 hardware, together with a
missing distinction in the common expression elimination / aliasing analysis.
In detail:

* Incorrect modelization of the x87 hardware: The compiler seems to assume that
    flds MEM_LOCATION_1
    fsts MEM_LOCATION_2
  will result in MEM_LOCATION_2 having the same value as MEM_LOCATION_1. This
is wrong;
  this is not how the x87 hardware behaves. The actual result is:
    *MEM_LOCATION_2 = convert_snan_to_qnan(*MEM_LOCATION_1).

* In the common expression elimination / aliasing analysis, the compilers seems
to keep
  track of a set of memory locations MEM_LOCATION_1, ..., MEM_LOCATION_n which
have the
  same value. In fact, this set needs to be partitioned into two sets: a subset
which
  contains the same value, and the complementary subset which contains
  convert_snan_to_qnan(value).

  In other words, each element of the set needs to be annotated with a bit that
tells
  whether the value has been subject to the convert_snan_to_qnan.