https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67183
Bug ID: 67183
Summary: Darwin stub vs. non_lazy pointer ordering incompatible
with clang assembler.
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jay.krell at cornell dot edu
Target Milestone: ---
Darwin stub vs. non_lazy pointer ordering incompatible with clang assembler.
Incorrect code results when targeting 32bit older than 10.5.
Request to output in the same order as clang for compatibility -- all stubs
before all non-lazy pointers.
This is really obscure.
It did hit me and I spent a long while debugging it, though still not fully.
There appears to be a bug in the LLVM assembler.
gcc by default does not run it.
i.e.:
gcc -S foo.c
clang foo.s
usually works, but hits a problem here.
It probably only occurs when targeting less than 10.5.
i.e. gcc -mmacosx-version-min=10.4
That switch works with both gcc and clang.
gcc outputs a presumably valid mix of stubs and non-lazy pointers.
You only get the stubs for earlier than 10.5.
clang front end outputs all stubs before all the non-lazy pointers.
gcc outputs them in a jumbled order. Presumably that is ok.
The clang assembler gets confused matching things up when there are non-lazy
pointers before stubs.
The bug is also 32bit specific as 64bit never has stubs.
Here is the bug in action:
jair:~ jay$ cat 1.c
void F1(int);
void F2(int);
extern int i;
void F3() { F1(i); }
void F4() { F2(i); }
jair:~ jay$ $HOME/gcc-5.2.0/bin/gcc -mmacosx-version-min=10.4 -m32 1.c -fPIC
-c -S
jair:~ jay$ clang -c 1.s -m32
jair:~ jay$ otool -tVv 1.o
1.o:
(__TEXT,__text) section
_F3:
00000000 pushl %ebp
00000001 movl %esp, %ebp
00000003 subl $0x8, %esp
00000006 calll ___x86.get_pc_thunk.ax
0000000b leal 86-11(%eax), %eax
00000011 movl _F3(%eax), %eax
00000013 movl _F3(%eax), %eax
00000015 subl $0xc, %esp
00000018 pushl %eax
00000019 calll 0x4c ## symbol stub for: _F1
0000001e addl $0x10, %esp
00000021 nop
00000022 leave
00000023 retl
_F4:
00000024 pushl %ebp
00000025 movl %esp, %ebp
00000027 subl $0x8, %esp
0000002a calll ___x86.get_pc_thunk.ax
0000002f leal 86-47(%eax), %eax
00000035 movl _F3(%eax), %eax
00000037 movl _F3(%eax), %eax
00000039 subl $0xc, %esp
0000003c pushl %eax
0000003d calll 0x51 ## symbol stub for: _i
00000042 addl $0x10, %esp
00000045 nop
00000046 leave
00000047 retl
Where is says "symbol stub for: _i", it should for _F2.
Having gcc output the stubs/non-lazy pointers in the same order as clang
in order to be compatible with the clang assembler might be nice.
For my part, I will either run non-LLVM assembler, or target 10.5 or newer,
at least until I develop or get a patch for gcc to reorder.
Thank you,
- Jay
