------- Comment #2 from pinskia at gmail dot com 2010-05-14 13:10 -------
Subject: Re: New: Unneeded +0.0 for c = 0.0 ; c = c+ a*b
Sent from my iPhone
On May 14, 2010, at 2:18 AM, "tkoenig at gcc dot gnu dot org"
<gcc-bugzi...@gcc.gnu.org
> wrote:
> This code leads to the adding of 0.0, which is a nop. Any
> signalling should have been done previously.
It is not signalling that matters here but signed zero. 0.0 + -0.0 ==
0.0. So without the 0.0 +, you can get a negative zero.
>
> 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
>
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44134
