https://llvm.org/bugs/show_bug.cgi?id=22947
Bug ID: 22947
Summary: Less efficient encoding using direct object emission
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: MC
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
For the following source the code generated by direct object emission is less
efficient than outputting to .s. Using r232386.
$ cat decrement.c
int foo(int a) {
return --a;
}
$ clang -c decrement.c -o decrement.o
$ clang -S decrement.c -o decrement.s && clang -c decrement.s -o
decrement_via_s.o
$ objdump -d decrement.o
decrement.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <foo>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 89 7d fc mov %edi,-0x4(%rbp)
7: 8b 7d fc mov -0x4(%rbp),%edi
a: 81 c7 ff ff ff ff add $0xffffffff,%edi
10: 89 7d fc mov %edi,-0x4(%rbp)
13: 89 f8 mov %edi,%eax
15: 5d pop %rbp
16: c3 retq
$ objdump -d decrement_via_s.o
decrement_via_s.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <foo>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 89 7d fc mov %edi,-0x4(%rbp)
7: 8b 7d fc mov -0x4(%rbp),%edi
a: 83 c7 ff add $0xffffffff,%edi
d: 89 7d fc mov %edi,-0x4(%rbp)
10: 89 f8 mov %edi,%eax
12: 5d pop %rbp
13: c3 retq
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