https://llvm.org/bugs/show_bug.cgi?id=29068
Bug ID: 29068
Summary: Static TLS for i386: addend ignored
Product: lld
Version: unspecified
Hardware: PC
OS: other
Status: NEW
Severity: release blocker
Priority: P
Component: ELF
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Consider the following piece of code:
#include <stdio.h>
_Thread_local struct {
int x, y, z;
} var = { .x = 12, .y = 34, .z = 56 };
void print(int);
int main() {
print(var.y);
}
Compiling it for i686 will yield the following main() function (abbreviated):
main:
movl %gs:0, %eax
pushl var@NTPOFF+4(%eax)
calll print
addl $4, %esp
xorl %eax, %eax
retl
Now, changing the call to print() to var.z, we get:
main:
movl %gs:0, %eax
pushl var@NTPOFF+8(%eax)
calll print
addl $4, %esp
xorl %eax, %eax
retl
This is all good. Now this is where the interesting part starts. If we link it
into a simple executable, we always generate the following code (as in, it
doesn't matter if var.y or var.z is used):
00014750 <main>:
14750: 65 a1 00 00 00 00 mov %gs:0x0,%eax
14756: ff b0 dc ff ff ff pushl -0x24(%eax)
1475c: e8 0f 00 00 00 call 14770 <print>
14761: 83 c4 04 add $0x4,%esp
14764: 31 c0 xor %eax,%eax
14766: c3 ret
The value that is passed to print() is equal to twelve. In other words, the
linker completely ignores the addend of the relocation, always making it use
the first element in the structure.
I'll debug this issue in more detail. Marking this as a blocker for LLVM 3.9,
as it makes PIE effectively broken when used in non-PIE code on i386. The issue
is both present in trunk and 3.9rc1.
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