http://llvm.org/bugs/show_bug.cgi?id=8318
Summary: ARM EABI: inefficient code saving/restoring sp around
function call
Product: libraries
Version: trunk
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: ARM
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
ToT & built both llvm-gcc and clang for arm-unknown-eabi target.
This simple C program:
--8<--
int foo(int a);
int
bar (int a)
{
return 5 + foo(a);
}
--8<--
Get compiled with clang as:
$ clang -ccc-host-triple armv7-unknown-eabi -S -O3 -o test1.S test1.c && cat
test1.S
--8<--
.syntax unified
.cpu cortex-a8
.eabi_attribute 10, 2
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.file "test1.c"
.text
.globl bar
.align 2
.type bar,%function
bar:
push {r11, lr}
bl foo
mov r11, sp <== ???
add r0, r0, #5
mov sp, r11 <== ???
ldmia sp!, {r11, pc}
.Ltmp0:
.size bar, .Ltmp0-bar
--8<--
I.e. the save & restore of sp is very odd and unnecessary IMHO.
When using llvm-gcc I don't get that output:
$ arm-unknown-eabi-gcc -mcpu=cortex-a8 -O3 -S -O3 -o test1.S test1.c && cat
test1.S
--8<--
.syntax unified
.cpu cortex-a8
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.file "test1.c"
.text
.globl bar
.align 2
.type bar,%function
bar:
push {r11, lr}
bl foo
add r0, r0, #5
ldmia sp!, {r11, pc}
.Ltmp0:
.size bar, .Ltmp0-bar
.ident "GCC: (GNU) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build)"
--8<--
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