Consider the following code:
int (*indirect_func)();
int indirect_call()
{
return indirect_func();
}
gcc 4.4.0 generates the following with -O2 -mcpu=cortex-a8 -S:
indirect_call:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
movw r3, #:lower16:indirect_func
stmfd sp!, {r4, lr}
movt r3, #:upper16:indirect_func
mov lr, pc
ldr pc, [r3, #0]
ldmfd sp!, {r4, pc}
The problem is that the instruction "ldr pc, [r3, #0]" is not considered a
function call by the Cortex-A8's branch predictor, as noted in DDI0344J section
5.2.1, Return stack predictions. Thus, the return from the called function is
mispredicted resulting in a penalty of 13 cycles compared to a direct call.
Rather than doing
mov lr, pc
ldr pc, [r3]
it should instead use the blx instruction as so:
ldr lr, [r3]
blx lr
which is considered a function call by the branch predictor, and has an
overhead of only one cycle compared to a direct call.
gcc -v:
Using built-in specs.
Target: arm-none-linux-gnueabi
Configured with: ../gcc-4.4.0/configure --target=arm-none-linux-gnueabi
--prefix=/usr/local/arm --enable-threads
--with-sysroot=/usr/local/arm/arm-none-linux-gnueabi/libc
Thread model: posix
gcc version 4.4.0 (GCC)
--
Summary: GCC generates suboptimal code for indirect function
calls on ARM
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: lessen42 at gmail dot com
GCC host triplet: i386-apple-darwin
GCC target triplet: arm-none-linux-gnueabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40887
