llvm dev - Apr 2006 - [LLVMdev] floating point exception and SSE2 instructions

On Thu, 20 Apr 2006, Simon Burton wrote:
>>> From what I remember, this is a bug in debian libc:
>> some floating point flags are set incorrectly causing SIGFPE.
>> Can't find the bug report ATM.
>
> Oh, it just showed up on numpy-discussion:
> http://sources.redhat.com/bugzilla/show_bug.cgi?id=10
>
> """
> #include <fenv.h>
> void feclearexcept(int ex)
>
> This function should clear the specified exception status bits in the 
> FPU status register. For CPUs with SSE support it should also clear the 
> MXCSR status register bits.
>
> The problem is that feclearexcept() clears the status control bits also,
> causing future floating-point errors to generate interrupts which will
> lead to a SIGFPE signal which terminates the program (unless caught by a
> SIGFPE handler).
> """
I don't see what this has to do with anything, but...
> Is there a way I can disable SSE instruction generation in LLVM ?
Yes.  Pass -mattr=-sse1,-sse2,-sse3 to lli or llc.

If you've linked the JIT into your app, you can specify this by calling 
cl::ParseCommandLineOptions on an static array, something like:

int argc;
char *Args[] = { "", "-mattr=-sse1,-sse2,-sse3", 0 };
cl::ParseCommandLineOptions(argc, Args, 0);

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

On Wed, 19 Apr 2006 18:21:32 -0500 (CDT)
Chris Lattner <sabre at nondot.org> wrote:
> 
> 
> I don't see what this has to do with anything, but...
Me neither.
> 
> > Is there a way I can disable SSE instruction generation in LLVM ?
> 
> Yes.  Pass -mattr=-sse1,-sse2,-sse3 to lli or llc.
Right, that fixed it.

BTW:

from the --help:
  -mattr=<a1,+a2,-a3,...>      - Target specific attributes (-mattr=help
for details)
  -mcpu=<cpu-name>             - Target a specific cpu type (-mcpu=help
for details)

but -mattr=help doesn't do anything.

Simon.


-- 
Simon Burton, B.Sc.
Licensed PO Box 8066
ANU Canberra 2601
Australia
Ph. 61 02 6249 6940
http://arrowtheory.com

On Thu, 20 Apr 2006, Simon Burton wrote:
> from the --help:
>  -mattr=<a1,+a2,-a3,...>      - Target specific attributes
(-mattr=help for details)
>  -mcpu=<cpu-name>             - Target a specific cpu type
(-mcpu=help for details)
>
> but -mattr=help doesn't do anything.
Annoyingly you have to specify a .bc file.  Try:

% llvm-as /dev/null -o dummy.bc
% llc -mattr=help dummy.bc

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/