https://llvm.org/bugs/show_bug.cgi?id=25722
Bug ID: 25722
Summary: ARM: integrated assembler should replace "ldr rX,
=imm" with "mov rX, #imm" if imm can be encoded as
immediate
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: ARM
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
gas does that, it produces smaller code, and it prevents follow-on "out of
range pc-relative fixup value" errors if the ldr is in a large-ish code block
and the offset to the constant pool is too large.
Example (reduced from boringssl):
thakis@thakis:~/src/chrome/src$ head arm.S
.fpu neon
.text
.align 4
.global foo
.hidden foo
.type foo, %function
ldr r4, =0
mov r1, r2
mov r1, r2
/* repeat previous instruction 2000 times here */
thakis@thakis:~/src/chrome/src$
third_party/llvm-build/Release+Asserts/bin/clang -c arm.S -march=armv7-a
-target arm-linux-androideab
arm.S:8:1: error: out of range pc-relative fixup value
ldr r4, =0
^
thakis@thakis:~/src/chrome/src$
third_party/android_tools/ndk/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin/arm-linux-androideabi-gcc
-c arm.S
thakis@thakis:~/src/chrome/src$
third_party/android_tools/ndk/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin/arm-linux-androideabi-objdump
-D -marm arm.o | head
arm.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <.text>:
0: e3a04000 mov r4, #0
4: e1a01002 mov r1, r2
8: e1a01002 mov r1, r2
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