https://bugs.llvm.org/show_bug.cgi?id=38620
Bug ID: 38620
Summary: ARM: Incorrect COFF relocation type for thumb bl
instruction
Product: libraries
Version: 6.0
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: Backend: ARM
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
LLVM generates incorrect relocation entry for thumb BL instruction, which
causes the instruction to be overwritten by a BLX instruction at load time.
In lib/Target/ARM/MCTargetDesc/ARMWinCOFFObjectWriter.cpp :
ARMWinCOFFObjectWriter::getRelocType(),
case ARM::fixup_t2_condbranch:
return COFF::IMAGE_REL_ARM_BRANCH20T;
case ARM::fixup_t2_uncondbranch:
return COFF::IMAGE_REL_ARM_BRANCH24T;
case ARM::fixup_arm_thumb_bl: <--- BLX23T is wrong relocation type for this
instruction
case ARM::fixup_arm_thumb_blx:
return COFF::IMAGE_REL_ARM_BLX23T;
fixup_arm_thumb_bl should return COFF::IMAGE_REL_ARM_BRANCH24T.
Here are the descriptions of the relocation types from
https://docs.microsoft.com/en-us/windows/desktop/debug/pe-format#arm-processors.
IMAGE_REL_THUMB_BRANCH24
0x0014
The instruction is fixed up with the 25-bit relative displacement to the 2-byte
aligned target. The least significant bit of the displacement is zero and is
not stored.This relocation corresponds to a Thumb-2 B instruction.
IMAGE_REL_THUMB_BLX23
0x0015
The instruction is fixed up with the 25-bit relative displacement to the 4-byte
aligned target. The low 2 bits of the displacement are zero and are not stored.
This relocation corresponds to a Thumb-2 BLX instruction.
This bug manifested when porting the rust compiler to thumbv7a-pc-windows-msvc.
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