Bug 149250 (IT_66328)

Created attachment 111265 [details]
cmc004.sh

Compilation script

Created attachment 111266 [details]
fdkcovcal6.f

Fortan source, subroutine

Created attachment 111267 [details]
check.f

Main routine

-O2 -fno-gcse cures this.  Will debug.

The bug is in strength reduction, which causes the loop computing:
      DO I=1,5
      DO J=1,5
      DS=0.D0
      DO K=1,5
      DS=DS+DCOVMAT(I,K)*DFT(K,J)
      ENDDO
      DF(I,J)=DS
      ENDDO
      ENDDO
to read DFT array members as if J in the second and following iteration was
one bigger than it actually should be.  So first iteration reads DFT(,1) but
second DFT(,3) and third DFT(,4).

As a workaround, -fno-automatic -fno-strength-reduce can be used.

Simplified testcase:

C Works with:
C -O1 -m32 -mcpu=i486 -fno-automatic
C -O2 -m32 -mcpu=i486
C -O2 -m32 -mcpu=i486 -fno-automatic -fno-strength-reduce
C Fails with:
C -O2 -m32 -mcpu=i486 -fno-automatic
        SUBROUTINE FOO(D)
        REAL*8 A,B,C,D,E
        DIMENSION A(5,5),B(5,5),C(5,5),D(5,5)
        DO I=1,5
          DO J=1,5
            A(I,J)=J
            B(I,J)=1
          ENDDO
        ENDDO
        DO I=1,5
          DO J=1,5
            E=0.D0
            DO K=1,5
              E=E+B(I,K)*A(K,J)
            ENDDO
            C(I,J)=E
          ENDDO
        ENDDO
        DO I=1,5
          DO J=1,5
            D(I,J)=C(I,J)
          ENDDO
        ENDDO
        END

        REAL*8 D
        DIMENSION D(5,5)
        CALL FOO(D)
        DO I=1,5
          DO J=1,5
            IF (D(I,J).NE.5*J) CALL ABORT
          ENDDO
        ENDDO
        END

Looking at loop_givs_reduce changes in GCC 3.3 I found
http://gcc.gnu.org/ml/gcc-patches/2002-09/msg00045.html
that indeed fixes this testcase.
In pseudo patch, the change that loop_givs_reduce was doing in this case when
reducing GIV 136, creating new pseudo 185, is:
 (note 134 484 298 NOTE_INSN_LOOP_BEG)
 (code_label 298 134 436 26 "" "" [1 uses])
 ...
 (insn 587 150 152 (set (reg:SI 136) (const_int 1 [0x1])) 45 {*movsi_1} (nil))
 ...
+(insn 876 853 881 (set (reg:SI 185) (const_int 2 [0x2])) -1 (nil) (nil))
 ...
 (note 153 492 279 NOTE_INSN_LOOP_BEG)
 (code_label 279 153 437 25 "" "" [1 uses])
 ...
 (insn 730 729 731 (parallel[(set (reg:SI 92) (ashift:SI (reg:SI 136) (const_int
2 [0x2]))) (clobber (reg:CC 17 flags))]))
 (insn 731 730 734 (parallel[(set (reg:SI 93) (plus:SI (reg:SI 92) (reg:SI 136)))
(clobber (reg:CC 17 flags))]) -1 (nil)
     (expr_list:REG_EQUAL (mult:SI (reg:SI 136) (const_int 5 [0x5])) (nil)))
 ...
 (note 177 744 240 NOTE_INSN_LOOP_BEG)
 (code_label 240 177 438 24 "" "" [1 uses])
 ...
 (note 230 228 237 NOTE_INSN_LOOP_CONT)
 ...
 (note 506 239 180 NOTE_INSN_LOOP_VTOP)
 ...
 (note 245 182 738 NOTE_INSN_LOOP_END)
 ...
 (insn 256 443 258 (parallel[(set (reg:SI 106) (ashift:SI (reg:SI 136) (const_int
2 [0x2]))) (clobber (reg:CC 17 flags))]))
 (insn 258 256 262 (parallel[(set (reg:SI 107) (plus:SI (reg:SI 106) (reg:SI
136))) (clobber (reg:CC 17 flags))])
     207 {*addsi_1} (nil) (expr_list:REG_EQUAL (mult:SI (reg:SI 136) (const_int 5
[0x5])) (nil)))
 ...
 (note 269 267 276 NOTE_INSN_LOOP_CONT)
 (insn 276 269 584 (parallel[(set (reg:SI 113) (plus:SI (reg:SI 136) (const_int 1
[0x1]))) (clobber (reg:CC 17 flags))]))
+(insn 873 276 879 (parallel[(set (reg:SI 185) (plus:SI (reg:SI 185) (const_int 1
[0x1]))) (clobber (reg:CC 17 flags))]))
-(insn 584 276 278 (set (reg:SI 136) (reg:SI 113)) 45 {*movsi_1} (nil) (nil))
+(insn 584 914 278 (set (reg:SI 136) (reg:SI 185)) -1 (nil) (nil))
 ...
 (note 498 278 156 NOTE_INSN_LOOP_VTOP)
 ...
 (note 284 158 286 NOTE_INSN_LOOP_END)
 ...
 (note 288 286 449 NOTE_INSN_LOOP_CONT)
 ...
 (note 490 297 137 NOTE_INSN_LOOP_VTOP)
 ...
 (note 303 139 305 NOTE_INSN_LOOP_END)

Here, tv->insn is insn 584.  Without Jan's patch the new increment insn (873)
is inserted before that instruction, and as the initial value for pseudo 185
is 2, not 1 (pseudo 136's initial value (1) * mult_val (1) + add_val (1)), it
means pseudo 136 has value 1 in the first iteration, but 3 in the second, 4
in the third etc.

Created attachment 116917 [details]
gcc32-rh149250.patch

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHBA-2005-660.html