[LLVMdev] [RFC] Identifying access to errno

[David Majnemer]

On Sat, Nov 23, 2013 at 9:59 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Renato Golin" <renato.golin at linaro.org>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "LLVM" <llvmdev at cs.uiuc.edu>
> > Sent: Saturday, November 23, 2013 10:53:09 AM
> > Subject: Re: [LLVMdev] [RFC] Identifying access to errno
> >
> >
> >
> >
> > On 23 November 2013 14:14, Hal Finkel < hfinkel at anl.gov > wrote:
> >
> >
> > On some systems (Linux/glibc, for example), some libm math functions
> > (like cos(double)) might set errno. It is important that we model
> > this, in general, to prevent miscompilation (we would not, for
> > example, want to reorder a call to cos in between a call to open and
> > a call to perror). However, almost no code in the wild checks errno
> > after calls to libm math functions, and this errno-setting behavior
> > prevents vectorization and other useful loop optimizations, CSE,
> > etc. Also, currently, the scalar llvm.<libm function> intrinsics are
> > subtly broken on systems where the underlying libm functions may set
> > errno, because the intrinsics are readonly, and may be implemented
> > by calls to the libm function (which might set errno), exposing us
> > to reordering problems (as in the example above).
> >
> >
> >
> > Hi Hal,
> >
> >
> > I'm confused. On one hand you're proposing us to stop reordering libm
> > calls because they might set errno (I agree with this), but on the
> > other hand you're saying that nobody cares and that prevents
> > optimizations (not sure I agree with this).
> >
>
> What I'm saying is that very few people actually check errno after libm
> calls, and so we're often preventing vectorization for no good reason.
> However, obviously we still need to prove that no errno access is occurring
> if we want to vectorize (unfortunately, our ability to do this may be
> limited outside of an LTO context -- but under fast-math or with some
> pragma, etc. we may be able to change the default assumptions). In short,
> I'm proposing that we both:
>
>  1. Be more strict to prevent unwanted reorderings (by actually modeling
> that these functions may *write* to errno).
>
>  2. Improve our modeling of errno so that we can ignore said writes
> (safely) when we know that value of errno is unused. A setting like
> -fno-math-errno should not "remove" the modeling of these writes, just
> declare our disinterest in the resulting value.
>
> But, what I'm trying to establish here is: how can we recognize possible
> errno access so that explicitly modeling the writes to errno does not
> unduly pessimize the surrounding code.
>
> >
> >
> >
> >
> > 1. Assume that all unknown external functions might read/write errno
> >
> > 2. Assume that all i32 pointers might point to errno (although we
> > might be able to do better by somehow leveraging TBAA for "int"?)
> >
> >
> >
> > Something like "MayBeErr", "IsErr", "IsntErr".
>
> On what?
>
> >
> >
> >
> >
> >
> > Does anyone see any problems with making stronger (type-based)
> > assumptions re: errno (and, thus, on what things may alias with
> > calls to errno-setting-libm functions)?
> >
> >
> > I don't, but I'm trying to think of a way to disable it if we know
> > it's "ok". Maybe -unsafe-math or something similar could disable
> > this pass, because it is expensive and will impact generated code.
> >
>
> We already run IPO/FunctionAttrs, and if we're conservative about escape,
> it would not add any additional expense. (Ff we do top-down propagation for
> static functions (or more-generally for LTO), then that could add overhead).
>
> >
> >
> >
> >
> > What if, for globals, we insisted that the global be named "errno"?
> >
> >
> >
> > I wouldn't be surprised if there was a system where the golbal error
> > is not errno. Windows maybe?
>
> Okay. We should check.
>

A Windows program can have two different global error numbers: Win32 and
C-Runtime flavor.

The Win32 flavor is exposed via GetLastError/SetLastError[Ex]
The CRT flavor (errno) is, currently, exposed via a macro that expands to
(*_errno())


>
> Thanks again,
> Hal
>
> >
> >
> > --renato
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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