[Bug middle-end/82004] [8 Regression] SPEC CPU2017 628.pop2_s miscompare

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82004

--- Comment #43 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to john henning from comment #42)
> A comment, an invitation, and some questions from one of those "SPEC people".
> 
> The comment: 
> 
> SPEC CPU starts from real applications (rather than artificial kernels). 
> Therefore, yes, it begins with code that includes practices that are less
> than perfect - including standards violations and mistakes in numerical
> methods.  
> 
> SPEC then weeds out benchmark candidates when they are discovered to be
> inadequate.  Many have been weeded out due to numerical malpractice, when
> they cannot be relied upon to produce correct answers within some predefined
> tolerance.  
> 
> The remaining candidates get pushed toward the standard, get pushed toward
> better numerical practices.
> 
> But all this is driven by testing.   If there's nobody pushing hard on
> compiler X advanced optimization feature Y, and if Y quietly relies on
> source code to be virtuous as regards Z, it is entirely possible that sins
> against virtue Z will escape notice until after release of a SPEC benchmark
> suite.
> 
> Pre-release, fixes are easy to sponsor and apply.   Post-release, it is much
> harder, because of SPEC's preference for "no moving targets". 
> 
> The invitation:
> 
> Thus I would invite any of the individuals who commented about "those SPEC
> people" to consider becoming one, and helping with the testing of benchmark
> candidates for the next benchmark suite.  SPEC has a mechanism to accept
> free volunteer labor :-) and if you would like to volunteer, please write to
> info at SPEC dot org.  If you like, you could mention my name and this bug
> when you do.

Thanks.  But all testing effort is limited.  We do test SPEC before the
release (but I am not aware that sources are available freely before
release - I mean, including the data and scripts).  But the issue required
flags to be reproduced that we didn't test in time.  A full review of
the source packages is of course not possible.

> The questions:
> 
> (q1) Do I correctly gather that the fix described here would have gone into
> GCC 8?

Yes.

> (q2) On aarch64 with GCC 8.2, GCC 9.3, and GCC 10.1 (Red Hat 7.8, glibc
> 2.17-307.0.2.el7.1) I am seeing the following 628.pop2_s miscompare and
> abort.  Is this the same problem or a different problem as the one described
> in this bug?
> 
> Miscompare:
>    1513:    Chlorophyll transmission table computed
>            Could not find range for chlamnt =  1.0000E-03
> 
> Abort: 
> 
>    Contents of pop2_s.err
>    ****************************************
>    Note: The following floating-point exceptions are signalling:
> IEEE_UNDERFLOW_FLAG
>    ------------------------------------------------------------------------
>    POP aborting...
>      set_chl_trans range error for chlamnt

It looks like the same issue, yes.

> (q3) The text mentions that -funsafe-math-optimizations as a workaround. 
> Was this tested?  Was it narrowed to any of the components of
> -funsafe-math-optimizations which apparently are -fno-signed-zeros,
> -fno-trapping-math, -fassociative-math and -freciprocal-math?  
> 
> I am just about to start such testing on my aarch64 system, but if someone
> has already done such, it would be useful to know.

I'm not aware that anyone tried to narrow down a workaround.

> (q4) *Is* there a proposed patch for 628.pop2_s?   Although such may be
> unlikely to be applied to CPU 2017, it is conceivable that there might be an
> updated version of 628.pop2_s in a future suite.  So, it would be good to
> have the proposed patch on record.

>From what I understand the proposed workaround is something like the
following if not rewriting the problematic loop in another way.

diff --git a/benchspec/CPU/628.pop2_s/src/sw_absorption.F90
b/benchspec/CPU/628.pop2_s/src/sw_absorption.F90
index 1aa25904..489e88f2 100644
--- a/benchspec/CPU/628.pop2_s/src/sw_absorption.F90
+++ b/benchspec/CPU/628.pop2_s/src/sw_absorption.F90
@@ -134,7 +134,7 @@

    real (r8), parameter, dimension(nchl) :: &!chl for table look-up
      chlcnc = (/                            &
-     .001_r8, .005_r8, .01_r8,  .02_r8,     &
+     .00099999999_r8, .005_r8, .01_r8,  .02_r8,     &
      .03_r8,  .05_r8,  .10_r8,  .15_r8,     &
      .20_r8,  .25_r8,  .30_r8,  .35_r8,     &
      .40_r8,  .45_r8,  .50_r8,  .60_r8,     &