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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Potential bad code generation with TLS and PowerPC"
href="https://llvm.org/bugs/show_bug.cgi?id=23108">23108</a>
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<th>Summary</th>
<td>Potential bad code generation with TLS and PowerPC
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>3.5
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>matt.davis@pgroup.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Hello,
I have discovered a potential inconsistency with LLVM and TLS code generation
for PowerPC, specifically Power8.
This test program exists as two object files,
test.o which references an external TLS variable "y", which belongs to
test_a.o.
These two .o files are linked together to create my test application. I am not
compiling this example with -relocation-model=pic
The variable in question, "y", is located in the initialized data section of
the
resulting executable. Now, on to my issue:
I have an external TLS integer variable, "y" which is declared as the following
from my test.c source file.
@y = external thread_local global i32 ,section ".comm" , align 4
When my code first tries to access this external variable, my compiler emits
the following
code sequence in llvm to access and then add the value of another int to 'y':
Build:
llc ./test.ll -mcpu=native -o ./test.s
Result:
%41 = load i32* @y, align 4, !dbg !20
...
%57 = add i32 %41, %56, !dbg !20
This generates the following asm:
Build:
/usr/bin/as ./test.s -mpower8 -o test.o
Result:
ld 0, y@got@tprel@l(6)
addis 7, 2, x@got@tprel@ha
lwz 30, -120(1)
lwz 29, -116(1)
add 28, 0, y@tls
lwz 0, -96(1)
lwz 28, 0(28)
ld 27, x@got@tprel@l(7)
lwz 26, -76(1)
lwz 25, -80(1)
lwz 24, -84(1)
lwz 23, -88(1)
add 26, 26, 25
Since GNU-ld can manipulate TLS instructions (for optimization purposes) the
following is the resulting object code after linking:
<+360>: addis r7,r13,0
<+364>: lwz r8,-112(r1)
<+368>: lwz r11,-124(r1)
<+372>: addi r7,r7,-28664
<+376>: lwz r12,0(r7)
<+380>: addis r0,r13,0
<+384>: nop
<+388>: lwz r30,-120(r1)
<+392>: lwz r29,-116(r1)
<+396>: li r28,-28668
<+400>: lwz r0,-96(r1)
<+404>: lwz r28,0(r28)
What concerns me, and is causing a segfault is offset +396 to +400.
The linker transforms the asm to object code:
add 28, 0, y@tls transformed--> li r28,-28668
The following lwz at +404 in the object code will try to load r28 (which is a
negative value/not-an-address). This segfaults my application.
Looking at the Power8 ABI for TLS and the specific "add 28, 0, y@tls"
instruction.
1) This appears to be the following relocation: R_PPC64_TLS (y + 0)
2) The Power8 ABI specifies that the "add" and "r9" registers are to be used.
It is not clear to me if the explicit register values as noted in the ABI
have to be used, but it would seem that way.
I have noticed there appears to be a clobber of a register if I specify PIC
compilation.
So is there a code generation fault on behalf of LLVM, or is there something I
have missed?</pre>
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