Re: SPEC2000 252.eon generates invalid results when compiled -O3 with g++ 3.3

[Casey Leedom <casey_leedom at yahoo dot com>]

/ Date: Fri, 28 Mar 2003 02:21:56 +0100
| From:"Jan Hubicka" <jh at suse dot cz>
| 
| >   Okay, adding -fpmath=sse solved the problem.  Interestingly enough adding
| > "-march=k8 -mcpu=k8" wasn't good enough.  I would have thought that it
| > would have been from reading gcc/config/i386/i386.c:override_options() ...
| 
| We've been chatting about this with glibc folks and they don't like the
| idea of losing the extra precisity 80bit temrporaries give, so
\ -mfpmath=sse can't be the default, unforutnately :(

  Okay, I understand that.  So let me see if I'm 100% on the same page (I'm
very new to the x86 instruction set and it's history):

 1. x87 is a stack-based floating point co-processor (originally on a separate
    chip).  It performs calulations in 80-bit arithmetic for "double"s?

 2. SSE is a register-file floating point engine.  It was designed to support
    digital media applications and performs 64-bit arithmetic for "doubles"?

 3. SSE2 is an update of SSE that extends the arithmetic to use 128-bit
    (80-bit?) arithmetic for "doubles"?

I'm a bit confused because SPEC2000 252.eon fails with x87 in 32-bit mode but
succeeds with -mfpmath=sse in 32-bit mode.  Unfortunately, 301.apsi fails with
SSE in 32-bit mode.  And then of course, they all succeed in 64-bit mode which
defaults to SSE/SSE2 ...

  Which brings us to a different issue: the default math mode is dependent on
the 32- vs. 64-bit compilation mode rather than the fundamental CPU
architecture capabilities.  For instance, here's the relevant portion of
gcc/config/i386/i386.c:override_options():

  if (TARGET_64BIT)
    {
      if (TARGET_ALIGN_DOUBLE)
        error ("-malign-double makes no sense in the 64bit mode");
      if (TARGET_RTD)
        error ("-mrtd calling convention not supported in the 64bit mode");
      /* Enable by default the SSE and MMX builtins.  */
      target_flags |= (MASK_SSE2 | MASK_SSE | MASK_MMX |
MASK_128BIT_LONG_DOUBLE);
      ix86_fpmath = FPMATH_SSE;
     }
  else
    ix86_fpmath = FPMATH_387;

Worse yet, there's no way I can find to turn on SSE2 explicitly.  This is just
plain confusing and doesn't make sense.  If there are default math options that
vary by compilation, those defaults ought to be based on the target CPU
architecture, not the 32- vs. 64-bit compilation mode.


/ Date: Fri, 28 Mar 2003 12:35:22 +0100
| From:"Jan Hubicka" <jh at suse dot cz>
| 
| Also you may be interested in playing with hammer-3_3-branch GCC in case
| you are interested in hammer performance. It is 3.3 GCC with some of
| optimizations backported from mainline giving siginificantly better
| results.  It is used at SuSE so it should be well tested for major
| architectures.
| 
| At the moment mainline GCC still performs slightly better but the
\ difference is not too big.

  I think that this is what we're playing with.  We got the rpm's from AMD ...

Casey

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