https://bugs.llvm.org/show_bug.cgi?id=35686
Bug ID: 35686
Summary: [X86] long nop generation is guarded incorrectly
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: a...@lowrisc.org
CC: llvm-bugs@lists.llvm.org
The intent of the HasNopl code in X86AsmBackend seems to be that long nops
should _not_ be generated when the target CPU is "generic" or "i686" (among
others). Consider the following input (from test/MC/X86/x86_long_nop.s):
inc %eax
.p2align 5
inc %eax
This generates long nops when there is no explicit -mcpu argument, because the
empty cpu string isn't tested when setting HasNopl:
./bin/llvm-mc -filetype=obj -triple=i686-pc-linux-gnu
../test/MC/X86/x86_long_nop.s | ./bin/llvm-objdump -d -no-show-raw-insn -
<stdin>: file format ELF32-i386
Disassembly of section .text:
.text:
0: incl %eax
1: nopw %cs:(%eax,%eax)
10: nopw %cs:(%eax,%eax)
1f: nop
20: incl %eax
And short nops when explicitly passing -mcpu=generic:
./bin/llvm-mc -filetype=obj -triple=i686-pc-linux-gnu -mcpu=generic
../test/MC/X86/x86_long_nop.s | ./bin/llvm-objdump -d -no-show-raw-insn -
<stdin>: file format ELF32-i386
Disassembly of section .text:
.text:
0: incl %eax
1: nop
2: nop
3: nop
4: nop
5: nop
6: nop
7: nop
8: nop
9: nop
a: nop
b: nop
c: nop
d: nop
e: nop
f: nop
10: nop
11: nop
12: nop
13: nop
14: nop
15: nop
16: nop
17: nop
18: nop
19: nop
1a: nop
1b: nop
1c: nop
1d: nop
1e: nop
1f: nop
20: incl %eax
See also: https://reviews.llvm.org/D41349 which exposes this issue ensuring
that "generic" is always given as the CPU string rather than the empty string.
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