| Issue |
177422
|
| Summary |
Unable to represent relocation for `__stack_chk_guard` _expression_ when targeting `thumb-windows-gnu` with PIC
|
| Labels |
backend:ARM,
llvm:mc,
platform:windows
|
| Assignees |
|
| Reporter |
alexrp
|
In Zig, we've been having trouble building our module tests for `thumb-windows-gnu` due to this error:
```
error: <unknown>:0: Cannot represent this _expression_
```
I've narrowed this down to here:
https://github.com/llvm/llvm-project/blob/8b59338e9b7bbea9357ae40427f796a6e44d44fc/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFObjectWriter.cpp#L43-L58
It occurs on code such as this:
```
.def ubsan_rt.TypeDescriptor.isSigned;
.scl 3;
.type 32;
.endef
.p2align 1
.code 16
.thumb_func
ubsan_rt.TypeDescriptor.isSigned:
.Lfunc_begin5:
.cv_func_id 6
.cv_loc 6 5 37 0
.seh_proc ubsan_rt.TypeDescriptor.isSigned
push.w {r11, lr}
.seh_save_regs_w {r11, lr}
mov r11, sp
.seh_save_sp r11
sub sp, #16
.seh_stackalloc 16
.seh_endprologue
movw r0, :lower16:(.refptr.__stack_chk_guard-(.LPC5_0+4))
movt r0, :upper16:(.refptr.__stack_chk_guard-(.LPC5_0+4))
.LPC5_0:
...snip...
```
Some details from GDB:
```
(gdb) break ARMWinCOFFObjectWriter.cpp:43 if IsCrossSection
Breakpoint 4 at 0x7fffebe3fc30: file /home/alexrp/Sources/llvm/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFObjectWriter.cpp, line 47.
(gdb) r
...snip...
Thread 1 "zig" hit Breakpoint 4, (anonymous namespace)::ARMWinCOFFObjectWriter::getRelocType (this=this@entry=0x7f2d060, Ctx=...,
Target=..., Fixup=..., IsCrossSection=IsCrossSection@entry=true, MAB=...)
at /home/alexrp/Sources/llvm/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFObjectWriter.cpp:47
47 const MCAsmBackend &MAB) const {
(gdb) p Target
$3 = (const llvm::MCValue &) @0x7ffffffe7d40: {SymA = 0x840db50, SymB = 0x840dc08, Cst = -4, Specifier = 0}
(gdb) p Target.SymA->getName()
$4 = {static npos = 18446744073709551615, Data = "" ".refptr.__stack_chk_guard", Length = 25}
(gdb) p Target.SymB->getName()
$5 = {static npos = 18446744073709551615, Data = "" ".LPC5_0", Length = 7}
(gdb) p FixupObject
$6 = (const llvm::MCFixup &) @0x17ceb040: {Value = 0x840dc78, Offset = 8, Kind = 4037, PCRel = false, LinkerRelaxable = false}
```
Reproducer: https://files.alexrp.com/30923/behavior.bc
```
❯ llc -O0 behavior.bc -filetype obj --relocation-model pic
<unknown>:0: error: Cannot represent this _expression_
<unknown>:0: error: Cannot represent this _expression_
<unknown>:0: error: Cannot represent this _expression_
...snip...
```
It's worth noting that it compiles w/o `--relocation-model pic`:
```
❯ llc -O0 behavior.bc -filetype obj
❯ llvm-objdump -Dr behavior.obj | grep -C3 stack_chk_guard
126: 46eb mov r11, sp
128: b0c2 sub sp, #0x108
12a: f240 0300 movw r3, #0x0
0000012a: IMAGE_REL_ARM_MOV32T .refptr.__stack_chk_guard
12e: f2c0 0300 movt r3, #0x0
132: 681b ldr r3, [r3]
134: 681b ldr r3, [r3]
```
cc @mstorsjo because it seems you did some work in this area some years back.
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