Compile following function with options -Os -mthumb -march=armv5te
unsigned get_least_bits(unsigned value)
{
return value << 9 >> 9;
}
Gcc generates:
ldr r3, .L2
@ sp needed for prologue
and r0, r0, r3
bx lr
.L3:
.align 2
.L2:
.word 8388607
A better code sequence should be:
lsl r0, 9
lsr r0, 9
bx lr
It is smaller (without constant pool) and faster.
This transformation was done very early and we can see it in the first tree
dump shift.c.003t.original. Gcc thinks and with a constant is cheaper than two
shifts. It is not true for this case in thumb ISA. On the other hand if the
constant used to and is small, such as 7, it is definitely cheaper than two
shifts. So which method is better is highly depend on both the constant and the
target ISA. It is difficult to make a correct decision in the TREE level. Maybe
we should define a peephole rule to do it.
--
Summary: inefficient code to extract least bits from an integer
value
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: carrot at google dot com
GCC build triplet: i686-linux
GCC host triplet: i686-linux
GCC target triplet: arm-eabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40697
