[Bug driver/19848] New: "options passed" from -verbose-asm do not adequately reflect optimization

["Thomas dot Koenig at online dot de" <gcc-bugzilla at gcc dot gnu dot org>]

This caused me a lot of pain in trying to disable specific
optimizations with -O1.

$ gfortran -O1 -o dasum-1.s -S -fverbose-asm -fno-loop-optimize -fno-tree-sra
-fno-tree-ter -fno-omit-frame-pointer -fno-tree-dse -fno-tree-dominator-opts
-fno-tree-ch -fno-tree-fre -fno-merge-constants -fno-cprop-registers
-fno-if-conversion2 -fno-defer-pop -fno-tree-lrs -fno-guess-branch-probability
-fno-tree-ccp -fno-tree-copyrename -fno-tree-dce -fno-if-conversion ../dasum.f
$ gfortran -O0 -o dasum-0.s -S -fverbose-asm  ../dasum.f

$ diff -u dasum-0.s dasum-1.s | head -30
--- dasum-0.s   2005-02-09 13:38:52.000000000 +0100
+++ dasum-1.s   2005-02-09 13:38:46.000000000 +0100
@@ -3,7 +3,12 @@
 // GNU F95 version 4.0.0 20050130 (experimental) (ia64-unknown-linux-gnu)
 //     compiled by GNU C version 4.0.0 20050130 (experimental).
 // GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
-// options passed:  -ffixed-form -auxbase-strip -O0 -fverbose-asm
+// options passed:  -ffixed-form -auxbase-strip -O1 -fverbose-asm
+// -fno-loop-optimize -fno-tree-sra -fno-tree-ter -fno-omit-frame-pointer
+// -fno-tree-dse -fno-tree-dominator-opts -fno-tree-ch -fno-tree-fre
+// -fno-merge-constants -fno-cprop-registers -fno-if-conversion2
+// -fno-defer-pop -fno-tree-lrs -fno-guess-branch-probability -fno-tree-ccp
+// -fno-tree-copyrename -fno-tree-dce -fno-if-conversion
 // options enabled:  -falign-loops -fargument-noalias-global
 // -fbranch-count-reg -fcommon -feliminate-unused-debug-types
 // -ffunction-cse -fgcse-lm -fident -fivopts -fkeep-static-consts
@@ -23,531 +28,280 @@
        .prologue 14, 35
        .save ar.pfs, r36
        alloc r36 = ar.pfs, 3, 4, 2, 0  //,,,,
+       adds r16 = -8, r12      //,,
        .vframe r37
        mov r37 = r12   //,
-       adds r12 = -80, r12     //,,
+       adds r12 = -32, r12     //,,
+       mov r17 = ar.lc //,
+       ;;
+       .savepsp ar.lc, 8
+       st8 [r16] = r17, 8      //,
        mov r38 = r1    //,

As you can see from the fact that there is no differences in
the "options enabled:" list in the assembly output, the assembly
should be identical.

However, the compilation results are very different, so -O1
seems to do other, undocumented things.

$ cat ../dasum.f
      double precision function dasum(n,dx,incx)
c
c     takes the sum of the absolute values.
c     jack dongarra, linpack, 3/11/78.
c     modified 3/93 to return if incx .le. 0.
c     modified 12/3/93, array(1) declarations changed to array(*)
c
      double precision dx(*),dtemp
      integer i,incx,m,mp1,n,nincx
c
      dasum = 0.0d0
      dtemp = 0.0d0
      if( n.le.0 .or. incx.le.0 )return
      if(incx.eq.1)go to 20
c
c        code for increment not equal to 1
c
      nincx = n*incx
      do 10 i = 1,nincx,incx
        dtemp = dtemp + dabs(dx(i))
   10 continue
      dasum = dtemp
      return
c
c        code for increment equal to 1
c
c
c        clean-up loop
c
   20 m = mod(n,6)
      if( m .eq. 0 ) go to 40
      do 30 i = 1,m
        dtemp = dtemp + dabs(dx(i))
   30 continue
      if( n .lt. 6 ) go to 60
   40 mp1 = m + 1
      do 50 i = mp1,n,6
        dtemp = dtemp + dabs(dx(i)) + dabs(dx(i + 1)) + dabs(dx(i + 2))
     *  + dabs(dx(i + 3)) + dabs(dx(i + 4)) + dabs(dx(i + 5))
   50 continue
   60 dasum = dtemp
      return
      end

-- 
           Summary: "options passed" from -verbose-asm do not adequately
                    reflect optimization
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: driver
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: Thomas dot Koenig at online dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19848