I have to compile real old legacy code written for the good old VAX:
it compiles fine without "-ff2c"
gfortran -DUNIX -DCTEXT -DF2C -DGLX11 -DX11 -DDX4 -DLITE -DWHATCHECK -pedantic -W -fno-automatic -ff2c -std=legacy -Waliasing -frepack-arrays -O0 -c a.f
a.f: In function ‘gvftor’:
a.f:37: internal compiler error: in expand_copysign, at optabs.c:2930
Please submit a full bug report,
with preprocessed source if appropriate.
See <URL:http://www.suse.de/feedback> for instructions.

Source follows:
      REAL FUNCTION GVFTOR(XYZ1,XYZ2,XYZ3,XYZ4)
C-------------------------------------------------------------------------------
C----                                                                       ----
C---- XYZ1 THROUGH 4 ARE THE COORDINATES OF FOUR POINTS IN SPACE. IF THESE  ----
C---- POINTS ARE THOUGHT CONNECTED, THEY DEFINE A TORSION ANGLE BETWEEN     ----
C---- THEM. THE FUNCTION RETURNS THIS TORSION ANGLE.                        ----
C----                                                                       ----
C-------------------------------------------------------------------------------
C===
      IMPLICIT      NONE
      REAL          XYZ1(3) ,XYZ2(3), XYZ3(3) ,XYZ4(3) ,BOND1(3),
     +              BOND2(3), BOND3(3), VPROD1(3), VPROD2(3),
     +              VPROD3(3), COSINE, GVFACS, GVFDP3
C----
C---- MAKE UP THE CONNECTING BONDS
C----
      CALL GVSTV3 (BOND1,XYZ2,XYZ1)
      CALL GVSTV3 (BOND2,XYZ3,XYZ2)
      CALL GVSTV3 (BOND3,XYZ4,XYZ3)
C----
C---- CALCULATE THE ANGLE
C----
      CALL GVSNM3 (BOND1)
      CALL GVSNM3 (BOND2)
      CALL GVSNM3 (BOND3)
      CALL GVSCP3 (VPROD1,BOND2,BOND1)
      CALL GVSCP3 (VPROD2,BOND3,BOND2)
      CALL GVSNM3 (VPROD1)
      CALL GVSNM3 (VPROD2)
      CALL GVSCP3 (VPROD3,VPROD1,VPROD2)
      COSINE=GVFDP3(VPROD1,VPROD2)
      GVFTOR=GVFACS(COSINE)
C----
C---- SET SIGN OF ANGLE FROM THE SIGN OF THE DOT PRODUCT
C---- OF VPROD3 AND BOND2.
C----
      GVFTOR=SIGN(GVFTOR,GVFDP3(VPROD3,BOND2))

      RETURN
      END