[Bug libgcc/71559] ICE in ix86_fp_cmp_code_to_pcmp_immediate, at config/i386/i386.c:23042 (KNL/AVX512)

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71559

--- Comment #11 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Fri, 17 Jun 2016, jakub at gcc dot gnu.org wrote:

> The patch is completely untested though (and wonder if we have testcases for
> not raising exceptions when isgreater etc. arguments are qNaNs.

We probably don't have such tests - tests for exception raising or its 
absence are fairly limited.  Those functions are covered in the glibc 
testsuite, but of course that only covers whichever insn patterns get used 
for __builtin_is* for that particular build of the glibc tests.

There are definitely bugs on some architectures involving ordinary ordered 
comparisons such as < and >= wrongly using quiet instructions.  See bug 
52451 for i386 (x87 floating point) and bug 58684 for powerpc, for 
example.  A consequence of this is that if you add tests of comparisons 
doing the right thing, some of those tests would immediately fail on some 
architectures.

(These sorts of local bugs with particular operations or optimizations 
being incorrect regarding exceptions are certainly easier to fix than the 
issues with optimizations not being aware of exceptions and rounding modes 
as extra inputs / outputs to floating-point operations.  The ones with 
individual operations could I expect largely be found through thorough 
test coverage; those with optimizations might be harder to find.)

Note that there is some ambiguity about whether LTGT RTL (and 
corresponding GENERIC / GIMPLE) should be a quiet operation corresponding 
to islessgreater, or ((x < y) || (x > y)) raising exceptions for quiet 
NaNs.  See the discussion at 
<https://gcc.gnu.org/ml/gcc-patches/2015-02/threads.html#00555>.  Fixing 
the ambiguity in either direction would probably involve changes to the 
part of the compiler expecting the other semantics.