[Bug tree-optimization/43716] [4.6 Regression] Revision 158105 miscompiles doduc.f90

["dominiq at lps dot ens dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #22 from dominiq at lps dot ens dot fr  2010-05-06 14:59 -------
> Have you been able to identify if there is an invalid optimization?

I have started to follow the spaghetti plate without success so far. However,
although I don't think the problem comes from any "invalid optimization", I
realized that '-funsafe-math-optimizations' (implied by -ffast-math) implies
'-freciprocal-math'. Thus with a vanilla trunk at revision 159085 (gfcp), I
tried:

gfcp -O3 -ffast-math -fno-reciprocal-math ../doduc.f90

which worked. Then I did the following change:

--- doduc.f90   2005-10-11 14:53:32.000000000 +0200
+++ doduc_nr.f90        2010-05-06 15:38:37.000000000 +0200
@@ -2378,13 +2378,13 @@
             dvg = DNUg(I)
             drf = -dvf*rf*rf
             drg = -dvg*rg*rg
-            rp = r*(dvf/vf-dvg/vg)
+            rp = r*(dvf*rf-dvg/vg)
             alo = .925*(vf/vg)**.239
             a = .67
             IF ( al.GE.alo ) a = .47
             vgj = VGJo(I)
             alop = alo*rp*.239/r
-            vbcrp = .25*vbcr*(dvf/vf-rp/(1.-r))
+            vbcrp = .25*vbcr*(dvf*rf-rp/(1.-r))
             alb = ALFb(I)
             alt = ALFt(I)
             tao = alop/(a*alo)

(since rf = 1./vf) and it worked too (note that it did not with only one change
in rp or vbcrp).

Now the questions are:
(1) why a change dealing with the propagation of minus signs interferes with
reciprocal-math?
(2) why moving VEC_safe_push from negate_value to eliminate_plus_minus_pair
trigger the miscompilation?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43716