[llvm] r176292 - Brag about function call vectorization in the docs.

[Benjamin Kramer]

On 01.03.2013, at 02:59, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
>> From: "Benjamin Kramer" <benny.kra at googlemail.com>
>> To: llvm-commits at cs.uiuc.edu
>> Sent: Thursday, February 28, 2013 1:33:46 PM
>> Subject: [llvm] r176292 - Brag about function call vectorization in the docs.
>> 
>> Author: d0k
>> Date: Thu Feb 28 13:33:46 2013
>> New Revision: 176292
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=176292&view=rev
>> Log:
>> Brag about function call vectorization in the docs.
>> 
>> Modified:
>>    llvm/trunk/docs/Vectorizers.rst
>> 
>> Modified: llvm/trunk/docs/Vectorizers.rst
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/Vectorizers.rst?rev=176292&r1=176291&r2=176292&view=diff
>> ==============================================================================
>> --- llvm/trunk/docs/Vectorizers.rst (original)
>> +++ llvm/trunk/docs/Vectorizers.rst Thu Feb 28 13:33:46 2013
>> @@ -245,6 +245,17 @@ See the table below for a list of these
>> |     |     | fmuladd |
>> +-----+-----+---------+
>> 
>> +The loop vectorizer knows about special instructions on the target
>> and will
>> +vectorize a loop containing a function call that maps to the
>> instructions. For
>> +example, the loop below will be vectorized on Intel x86 if the
>> SSE4.1 roundps
>> +instruction is available.
>> +
>> +.. code-block:: c++
>> +
>> +  void foo(float *f) {
>> +    for (int i = 0; i != 1024; ++i)
>> +      f[i] = floorf(f[i]);
>> +  }
> 
> Do some of these only happen in fast-math mode? floorf is a nice example because it will never set errno (according to the Linux man page), and so is always safe. Nevertheless, for functions that set errno I assume we only do this in fast-math mode and we should say so (unless, of course, I'm wrong about that -- but in that case, we might have a correctness problem).

We do this always. I audited all the libm function definitions in clang a while ago. Most of them set errno unless -fno-math-errno is specified (which is the default on BSDs). The rounding functions like floorf are specified to set EDOM in POSIX afaik, but there is no way that could occur with our floating point types, so we mark it as not setting errno.

One thing we could do with -ffast-math is vectorizing sqrt(). I avoided that for now because the llvm.sqrt intrinsic has undefined behavior for negative numbers.

> On that thought, we might want some kind of analysis that can determine if errno from some math function is unused so that we can optimize regardless of the conformance mode when possible.

That's tricky though, errno is always a global variable but the way it's accessed varies from operating system to operating system.

- Ben
> 
> Thanks again,
> Hal
> 
>> 
>> Partial unrolling during vectorization
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> 
>> 
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