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Re: patch to fix rtl documentation for new floating point comparisons


On Tue, 17 Feb 2015, Richard Earnshaw wrote:

> So the problem we have today is the compiler has no way to distinguish
> between, say, < and __builtin_isless.  According to Annex F (c99) the
> former should be signalling while the latter quiet.

We do have a way: < is LT and __builtin_isless is !UNGE.

__builtin_islessgreater is !UNEQ.  The question is whether it's also LTGT 
or whether LTGT means LT || GT.  And the existing documentation of 
LTGT_EXPR leaves this unspecified, which seems clearly unhelpful.  Either 
way, you have existing code in GCC that's incorrect (i.e. that does not 
correspond to the set of transformations that are actually valid for the 
chosen semantics).

> I suspect there are two ways we could deal with that: add new comparison
> RTL codes to distinguish the cases; or use something like the RTL /v bit
> on a comparison to indicate that it should be signalling.
> 
> Of the two, the latter would probably be easiest to implement in a
> backwards compatible manner (backends not understanding /v would
> continue to use their existing code paths), but it would still take a
> fair amount of rejigging in the mid end to fully preserve the signalling
> nature of comparisons: there are many places where just RTX_CODE is
> available and a new pattern is generated from that.  The first method
> would require all back-ends to be updated pretty much simultaneously to
> handle the new RTL codes.

I don't know the optimal way of representing these variants in GENERIC, 
GIMPLE and RTL (the existing representation can cover everything, and is 
unambiguous apart from LTGT, but may not be optimal).

I think the main difficulty in proper Annex F support would be making 
optimizers (on each IR) understand the side-effects operations have in 
terms of raising exceptions, and how operations may take the rounding mode 
or existing exceptions raised as inputs - with an associated issue of 
defining the existing default floating-point rules well enough to keep a 
default mode that doesn't unduly inhibit optimization.

-- 
Joseph S. Myers
joseph@codesourcery.com


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