http://llvm.org/bugs/show_bug.cgi?id=17562
Bug ID: 17562
Summary: R_X86_64_TPOFF64 shows up in an invalid place
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
C testcase:
__thread int obj;
int test() {
return ((int)&obj) > 0;
}
The assembly for this produced by clang -O2 is:
# BB#0: # %entry
movabsq $obj@TPOFF, %rax
movq %fs:0, %rcx
addl %eax, %ecx
testl %ecx, %ecx
while the assembly produced by gcc -O2 is:
.LFB0:
.cfi_startproc
movq %fs:0, %rax
addq $obj@tpoff, %rax
testl %eax, %eax
I'll admit it's subtle, but that's a 32-bit immediate on the addq (addq will
sign extend to 64 bits if you give it a mere 32-bit immediate). This is
important because the ABI does not permit an R_X86_64_TPOFF64 relocation here,
that may only be used in conjunction with a GOT lookup:
"R_X86_64_TPOFF64 and R_X86_64_TPOFF32 resolve to the offset from
the thread pointer to a thread-local variable. The former is generated in
response
to R_X86_64_GOTTPOFF, that resolves to a PC-relative address of a GOT entry
containing such a 64-bit offset." - amd64 psABI 0.99.6
and the linker (gold) does indeed check this:
ld: error: y.o: unexpected reloc 18 in object file
y.o:y5.c:function test: error: unexpected reloc 18 in object file
when compiling the clang-produced .o file, but not the gcc-produced .o file.
(To be clear, this is not an error in the MC layer.)
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