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[Bug target/50856] New: ARM: suboptimal code for absolute difference calculation
- From: "siarhei.siamashka at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Mon, 24 Oct 2011 14:52:40 +0000
- Subject: [Bug target/50856] New: ARM: suboptimal code for absolute difference calculation
- Auto-submitted: auto-generated
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50856
Bug #: 50856
Summary: ARM: suboptimal code for absolute difference
calculation
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: siarhei.siamashka@gmail.com
gcc generates suboptimal code on ARM for "abs(a - b)" type of operation, which
is used for example in paeth png filter: http://www.w3.org/TR/PNG-Filters.html
Given the following test code:
int absolute_difference1(unsigned char a, unsigned char b)
{
return a > b ? a - b : b - a;
}
int absolute_difference2(unsigned char a, unsigned char b)
{
int tmp = a;
if ((tmp -= b) < 0)
tmp = -tmp;
return tmp;
}
The current gcc svn trunk (r180383) generates the following code for -O2 and
-Os optimizations:
.cpu arm10tdmi
.eabi_attribute 27, 3
.eabi_attribute 28, 1
.fpu vfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 2
.eabi_attribute 30, 4
.eabi_attribute 34, 0
.eabi_attribute 18, 4
.file "test.c"
.text
.align 2
.global absolute_difference1
.type absolute_difference1, %function
absolute_difference1:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
cmp r0, r1
rsbhi r0, r1, r0
rsbls r0, r0, r1
bx lr
.size absolute_difference1, .-absolute_difference1
.align 2
.global absolute_difference2
.type absolute_difference2, %function
absolute_difference2:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
rsb r0, r1, r0
cmp r0, #0
rsblt r0, r0, #0
bx lr
.size absolute_difference2, .-absolute_difference2
.ident "GCC: (GNU) 4.7.0 20111024 (experimental)"
.section .note.GNU-stack,"",%progbits
Even in the quite explicit second code variant ('absolute_difference2'
function), gcc does not generate the expected SUBS + NEGLT pair of
instructions. Also for ARMv6 capable processors even a single USAD8 instruction
could be used here if both operands are known to have values in [0-255] range
and if high latency of this instruction can be hidden.