[LLVMdev] [RFC] Identifying access to errno

[Hal Finkel]

----- 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.

Thanks again,
Hal

> 
> 
> --renato

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