[Bug target/37845] gcc ignores FP_CONTRACT pragma set to OFF

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

------- Comment #5 from joseph at codesourcery dot com  2008-10-16 16:31 -------
Subject: Re:  gcc ignores FP_CONTRACT pragma set to OFF

On Thu, 16 Oct 2008, rguenth at gcc dot gnu dot org wrote:

> Confirmed.  The FP_CONTRACT macro is not implemented, but the default behavior
> of GCC is to behave like it was set to OFF.

No, the default is to behave like it is ON, on targets with fused 
operations, with target-specific options to turn it off.  A default of ON 
is fine according to C99.

It so happens that GCC's version of "ON" is buggy, in that it contracts 
even outside the bounds of source language expressions - those boundaries 
are lost at gimplification, long before the insn patterns are applied.  
Fixing this would probably require fused operations to be identified in 
the front end in conforming mode, and the only insn patterns in conforming 
mode for the fused instructions to have RTL describing them precisely as 
fused operations so non-fused ones don't get combined.

This situation is much like the other floating-point pragmas:

* GCC doesn't support any of the pragmas, and doesn't claim to support 
those parts of C99; the features can only be enabled or disabled for a 
whole translation unit on the command line.

* The features for a whole translation unit are in practice rather buggy 
and incomplete.  -fno-cx-limited-range (on by default) doesn't work for 
constant arithmetic (bug 30789).  -ftrapping-math (on by default) doesn't 
cause all required exceptions to be generated, and spurious exceptions are 
generated in some cases; no real effort has been made to get it to work 
properly in all cases.  -frounding-math (off by default) comes with a 
specific warning in the manual that it's experimental and incomplete, 
which is quite correct.

Yes, there should be a target-independent option to disable contracting 
rather than needing separate options for each target, and in conformance 
mode (flag_iso) contracting, if enabled, should be restricted to source 
language expressions.  I'm not aware of anyone working on this, or on the 
other issues with the options approximating to the standard pragmas, or on 
the pragmas themselves.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37845