Re: GCC options and floating-point correctness (benchmarks)

[Dan Nicolaescu <dann at godzilla dot ics dot uci dot edu>]

Scott Robert Ladd <coyote@coyotegulch.com> writes:

  > Joseph S. Myers wrote:
  >> What if -ffloat-store is used in alternative to those?  It doesn't
  >> actually work as far as causing conformance (e.g., casts to same type
  >> still get discarded rather than reducing precision), but it is supposed to
  >> help.
  >
  > -ffloat-store and -mieee-fp don't change the results.

This is contradicted by my results. 

>From /proc/cpuinfo
model name      : Intel(R) XEON(TM) CPU 2.00GHz

gcc is gcc-3.2

wget http://www.netlib.org/paranoia/paranoia.c
gcc -O2 -ffloat-store paranoia.c -lm 
./a.out

The number of  DEFECTs  discovered =         1.
The number of  FLAWs  discovered =           1.

Without -ffloat-store: 

gcc -O2 paranoia.c -lm 

The number of  FAILUREs  encountered =       2.
The number of  SERIOUS DEFECTs  discovered = 3.
The number of  DEFECTs  discovered =         4.
The number of  FLAWs  discovered =           2.

With -mieee-fp:

gcc -O2 -mieee-fp paranoia.c -lm 

The number of  FAILUREs  encountered =       2.
The number of  SERIOUS DEFECTs  discovered = 3.
The number of  DEFECTs  discovered =         4.
The number of  FLAWs  discovered =           2.


Now, how about ICC-8.0? 

icc -O2 paranoia.c

The number of  FAILUREs  encountered =       2.
The number of  SERIOUS DEFECTs  discovered = 2.
The number of  DEFECTs  discovered =         3.
The number of  FLAWs  discovered =           1.

icc -march=pentium4 paranoia.c
paranoia.c(416) : (col. 2) remark: BLOCK WAS VECTORIZED.
[Note that vectorization was performed even though no optimization
flag was passed]

The number of  FLAWs  discovered =           1.
[I haven't checked if this result was due to some code being
optimized away at compile time. If anyone wants to check I can
provide the assembly.]


icc -march=pentiumiii paranoia.c
paranoia.c(416) : (col. 2) remark: BLOCK WAS VECTORIZED.

The number of  FAILUREs  encountered =       2.
The number of  SERIOUS DEFECTs  discovered = 4.
The number of  DEFECTs  discovered =         3.
The number of  FLAWs  discovered =           1.

icc -march=pentiumiii -O2 -mp1 paranoia.c
paranoia.c(416) : (col. 2) remark: BLOCK WAS VECTORIZED.

The number of  FAILUREs  encountered =       3.
The number of  SERIOUS DEFECTs  discovered = 3.
The number of  DEFECTs  discovered =         3.
The number of  FLAWs  discovered =           1.

The results for the Portland Group compiler version 5.0-2

pgcc -O2 paranoia.c 

The number of  FAILUREs  encountered =       3.
The number of  SERIOUS DEFECTs  discovered = 2.
The number of  DEFECTs  discovered =         3.
The number of  FLAWs  discovered =           2.

pgcc paranoia.c 
... the test hangs ... 


On sparc-sun-solaris2.7 all the tests pass with both gcc and cc.


Draw your own conclusions from this data. 

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