Re: [PATCH v3] Change default to -fno-math-errno

[Joseph Myers <joseph at codesourcery dot com>]

On Wed, 27 Jun 2018, Wilco Dijkstra wrote:

> Joseph Myers wrote:
> > On Tue, 26 Jun 2018, Wilco Dijkstra wrote:
> 
> > > That looks incorrect indeed but that's mostly a problem with -fmath-errno as it
> > > would result in GCC assuming the function is const/pure when in fact it isn't.
> > > Does ATTR_MATHFN_FPROUNDING imply that errno is dead after the call?
> >
> > No, it implies a pure function, so the previous errno value is still live 
> > after the call.
> 
> OK, so given GLIBC math functions may set errno anyway (irrespectively of the
> math-errno setting used), those 2 settings are not sufficient. Ideally you'd need
> to model GLIBC style errno as may or may not be set (still pure since it doesn't
> affect global state but not assuming errno is live afterwards). But perhaps it
> doesn't matter since this has been incorrect for years now...

Well, in some cases I think it can be treated as pure with 
-fno-math-errno, because that option implies functions may or may not set 
errno.  So (in the absence of spurious errno settings from libm functions 
not encountering errors), if the compiler reuses an errno value from 
before a libm function call because the function was marked as pure, 
that's equivalent to the function call not in fact setting errno.  The 
trouble is that (a) generating a read of errno after the call when the 
actual access in the abstract machine was before the call would be bad if 
that call in fact sets errno, and (b) each execution of a call in the 
abstract machine should either execute as if it set errno, or execute as 
if it did not set errno, and optimizations might not preserve that 
property.

-- 
Joseph S. Myers
joseph@codesourcery.com