https://llvm.org/bugs/show_bug.cgi?id=23098
Bug ID: 23098
Summary: Less efficient encoding of shl 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 a less efficient instruction encoding is generated for
direct object emission than outputting asm (with -via-file-asm). Using r232944.
This is the case as -O0/-O1/-O2/-O3.
$ cat test.c
int a;
void fn1() { a = a << 1 & 255; }
$ clang -c test.c -O3 -o test.o && objdump -d test.o
test.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <fn1>:
0: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 6 <fn1+0x6>
6: 83 e0 7f and $0x7f,%eax
9: c1 e0 01 shl $0x1,%eax
c: 89 05 00 00 00 00 mov %eax,0x0(%rip) # 12 <fn1+0x12>
12: c3 retq
$ clang -c test.c -O3 -via-file-asm -o tests.o && objdump -d tests.o
tests.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <fn1>:
0: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 6 <fn1+0x6>
6: 83 e0 7f and $0x7f,%eax
9: d1 e0 shl %eax
b: 89 05 00 00 00 00 mov %eax,0x0(%rip) # 11 <fn1+0x11>
11: c3 retq
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