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Date: Wed, 28 Jul 2004 19:56:00 -0000
From: "falk at debian dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20040728195603.16807.falk@debian.org>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug tree-optimization/16807] New: [lno] Weird loop unrolling
X-Bugzilla-Reason: CC
X-SW-Source: 2004-07/txt/msg03391.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

gcc version 3.5-tree-ssa-lno 20040719 (merged 20040718)

For this code:

int g (int *p, int n) {
    int r = 0, i;
    for (i = 0; i < n; ++i)
	r += p[i];
    return r;
}

with -O3 -funroll-all-loops, I get:

g:
	.frame $30,0,$26,0
	.prologue 0
	mov $31,$0
	ble $17,$L4
	lda $17,-1($17)
	mov $31,$24
$L5:
	ldl $23,0($16)
	zapnot $17,15,$3
	lda $2,1($24)
	lda $3,1($3)
	lda $24,7($2)
	lda $4,1($2)
	lda $5,2($2)
	lda $6,3($2)
	lda $7,4($2)
	lda $8,5($2)
	lda $22,6($2)
	cmpeq $2,$3,$2
	cmpeq $4,$3,$4
	cmpeq $5,$3,$5
	cmpeq $6,$3,$6
	cmpeq $7,$3,$7
	cmpeq $8,$3,$8
	cmpeq $22,$3,$22
	cmpeq $24,$3,$25
	addl $0,$23,$0
	lda $3,4($16)
	bne $2,$L4
	ldl $1,0($3)
	lda $16,28($3)
	addl $0,$1,$0
	bne $4,$L4
	ldl $1,4($3)
	addl $0,$1,$0
	bne $5,$L4
	ldl $1,8($3)
	addl $0,$1,$0
	bne $6,$L4
	ldl $1,12($3)
	addl $0,$1,$0
	bne $7,$L4
	ldl $1,16($3)
	addl $0,$1,$0
	bne $8,$L4
	ldl $1,20($3)
	addl $0,$1,$0
	bne $22,$L4
	ldl $1,24($3)
	addl $0,$1,$0
	beq $25,$L5
	.align 4
$L4:
	ret $31,($26),1
	.end g

which strikes me as an extremely weird way of unrolling a loop, and in fact
it takes twice the time as the code 3.5 generates, which is:

g:
	.frame $30,0,$26,0
	.prologue 0
	mov $17,$25
	ble $17,$L38
	lda $17,-1($17)
	cmple $25,1,$1
	ldl $0,0($16)
	lda $24,1($31)
	and $17,7,$17
	lda $23,4($16)
	bne $1,$L4
	addl $31,$17,$17
	beq $17,$L5
	cmpeq $17,1,$1
	bne $1,$L31
	cmpeq $17,2,$1
	bne $1,$L32
	cmpeq $17,3,$1
	bne $1,$L33
	cmpeq $17,4,$1
	bne $1,$L34
	cmpeq $17,5,$1
	bne $1,$L35
	cmpeq $17,6,$1
	bne $1,$L36
	ldl $1,4($16)
	lda $23,8($16)
	lda $24,2($31)
	addl $0,$1,$0
$L36:
	ldl $1,0($23)
	addl $24,1,$24
	lda $23,4($23)
	addl $0,$1,$0
$L35:
	ldl $1,0($23)
	addl $24,1,$24
	lda $23,4($23)
	addl $0,$1,$0
$L34:
	ldl $1,0($23)
	addl $24,1,$24
	lda $23,4($23)
	addl $0,$1,$0
$L33:
	ldl $1,0($23)
	addl $24,1,$24
	lda $23,4($23)
	addl $0,$1,$0
$L32:
	ldl $1,0($23)
	addl $24,1,$24
	lda $23,4($23)
	addl $0,$1,$0
$L31:
	ldl $2,0($23)
	addl $24,1,$24
	lda $23,4($23)
	cmple $25,$24,$1
	addl $0,$2,$0
	bne $1,$L4
$L5:
	ldl $1,0($23)
	ldl $8,4($23)
	addl $24,8,$24
	ldl $6,8($23)
	ldl $7,12($23)
	cmple $25,$24,$22
	ldl $4,16($23)
	ldl $5,20($23)
	addl $0,$1,$1
	ldl $2,24($23)
	ldl $3,28($23)
	lda $23,32($23)
	addl $1,$8,$1
	addl $1,$6,$1
	addl $1,$7,$1
	addl $1,$4,$1
	addl $1,$5,$1
	addl $1,$2,$1
	addl $1,$3,$0
	beq $22,$L5
$L4:
	ret $31,($26),1
$L38:
	mov $31,$0
	ret $31,($26),1
	.end g

-- 
           Summary: [lno] Weird loop unrolling
           Product: gcc
           Version: lno
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: falk at debian dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: alphaev68-unknown-linux-gnu
  GCC host triplet: alphaev68-unknown-linux-gnu
GCC target triplet: alphaev68-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16807