Currently, LLD does not support the complete set of ARM group relocations.
Given that I intend to start using these in the Linux kernel [0], let's add
support for these.
This implements the group processing as documented in the ELF psABI. Notably,
this means support is dropped for very far symbol references that also carry a
small component, where the immediate is rotated in such a way that only part of
it wraps to the other end of the 32-bit word. To me, it seems unlikely that
this is something anyone could be relying on, but of course I could be wrong.
[0] https://lore.kernel.org/r/20211122092816.2865873-8-ardb@kernel.org/
Reviewed By: peter.smith, MaskRay
Differential Revision: https://reviews.llvm.org/D114172
See http://lists.llvm.org/pipermail/llvm-dev/2020-April/140549.html
For the record, GNU ld changed to 64k max page size in 2014
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=7572ca8989ead4c3425a1500bc241eaaeffa2c89
"[RFC] ld/ARM: Increase maximum page size to 64kB"
Android driver forced 4k page size in AArch64 (D55029) and ARM (D77746).
A binary linked with max-page-size=4096 does not run on a system with a
higher page size configured. There are some systems out there that do
this and it leads to the binary getting `Killed!` by the kernel.
In the non-linker-script cases, when linked with -z noseparate-code
(default), the max-page-size increase should not cause any size
difference. There may be some VMA usage differences, though.
Reviewed By: psmith, MaskRay
Differential Revision: https://reviews.llvm.org/D77330
The R_ARM_ALU_PC_G0 and R_ARM_LDR_PC_G0 relocations are used by the
ADR and LDR pseudo instructions, and are the basis of the group
relocations that can load an arbitrary constant via a series of add, sub
and ldr instructions.
The relocations need to be obtained via the .reloc directive.
R_ARM_ALU_PC_G0 is much more complicated as the add/sub instruction uses
a modified immediate encoding of an 8-bit immediate rotated right by an
even 4-bit field. This means that the range of representable immediates
is sparse. We extract the encoding and decoding functions for the modified
immediate from llvm/lib/Target/ARM/MCTargetDesc/ARMAddressingModes.h as
this header file is not accessible from LLD. Duplication of code isn't
ideal, but as these are well-defined mathematical functions they are
unlikely to change.
Differential Revision: https://reviews.llvm.org/D75349
Ard Biesheuvel