[Bug middle-end/44134] New: Unneeded +0.0 for c = 0.0 ; c = c+ a*b

[tkoenig at gcc dot gnu dot org]

This code leads to the adding of 0.0, which is a nop.  Any
signalling should have been done previously.

i...@linux-fd1f:/tmp> cat mult.f90
subroutine foo(a,b,c)
  real, intent(in) :: a,b
  real, intent(out) :: c
  c = 0.0
  c = c + a*b
end subroutine foo
i...@linux-fd1f:/tmp> gfortran -O3 -fdump-tree-optimized -S mult.f90
i...@linux-fd1f:/tmp> cat mult.f90.142t.optimized

;; Function foo (foo_)

foo (real(kind=4) & restrict a, real(kind=4) & restrict b, real(kind=4) &
restrict c)
{
  real(kind=4) D.1542;
  real(kind=4) D.1541;
  real(kind=4) D.1540;
  real(kind=4) D.1539;

<bb 2>:
  D.1539_4 = *a_3(D);
  D.1540_6 = *b_5(D);
  D.1541_7 = D.1539_4 * D.1540_6;
  D.1542_8 = D.1541_7 + 0.0;
  *c_1(D) = D.1542_8;
  return;

}

i...@linux-fd1f:/tmp> cat mult.s 
        .file   "mult.f90"       
        .text                    
        .p2align 4,,15           
.globl foo_                      
        .type   foo_, @function  
foo_:                            
.LFB0:                           
        movss   (%rdi), %xmm0    
        mulss   (%rsi), %xmm0    
        addss   .LC0(%rip), %xmm0
        movss   %xmm0, (%rdx)    
        ret                      
.LFE0:                           
        .size   foo_, .-foo_     
        .section        .rodata.cst4,"aM",@progbits,4
        .align 4                                     
.LC0:
        .long   0
        .section        .eh_frame,"a",@progbits
.Lframe1:
        .long   .LECIE1-.LSCIE1
.LSCIE1:
        .long   0
        .byte   0x1
        .string "zR"
        .uleb128 0x1
        .sleb128 -8
        .byte   0x10
        .uleb128 0x1
        .byte   0x3
        .byte   0xc
        .uleb128 0x7
        .uleb128 0x8
        .byte   0x90
        .uleb128 0x1
        .align 8
.LECIE1:
.LSFDE1:
        .long   .LEFDE1-.LASFDE1
.LASFDE1:
        .long   .LASFDE1-.Lframe1
        .long   .LFB0
        .long   .LFE0-.LFB0
        .uleb128 0
        .align 8
.LEFDE1:
        .ident  "GCC: (GNU) 4.6.0 20100513 (experimental)"
        .section        .note.GNU-stack,"",@progbits


-- 
           Summary: Unneeded +0.0 for c = 0.0 ; c = c+ a*b
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tkoenig at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44134