[LLVMdev] [RFC] Identifying access to errno

[Hal Finkel]

----- Original Message -----
> From: "David Majnemer" <david.majnemer at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Renato Golin" <renato.golin at linaro.org>, "LLVM" <llvmdev at cs.uiuc.edu>
> Sent: Saturday, November 23, 2013 2:37:52 PM
> Subject: Re: [LLVMdev] [RFC] Identifying access to errno
> 
> 
> Oh, and I forgot a third "_doserrno" for which no amount of
> documentation lends itself to a consistent description of its
> behavior.

Are all of these things possibly set by cos(double) and friends?

 -Hal

> 
> 
> 
> On Sat, Nov 23, 2013 at 12:28 PM, David Majnemer <
> david.majnemer at gmail.com > wrote:
> 
> 
> 
> 
> 
> 
> 
> 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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> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory