[PATCH] D36507: [ARM] GlobalISel: Select globals in PIC mode

Peter Smith via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Wed Aug 9 08:36:28 PDT 2017

peter.smith added a comment.

A potential drawback of using MOVW and MOVT in ELF is that the maximum immediate that can be used is a signed 16-bit value due to the constraint in ARMELF Section

> For relocations processing MOVW and MOVT instructions (in both ARM and Thumb state), the initial addend is formed by interpreting the 16-bit literal field of the instruction as a 16-bit signed value in the range -32768 <= A < 32768. The interpretation is the same whether the relocated place contains a MOVW instruction or a MOVT instruction.

This is captured in https://bugs.llvm.org/show_bug.cgi?id=28229

I'm not familiar enough with ARMExpandPseudoInsts.cpp to know if it guarantees the addend won't overflow this range, or if you can select MOVT/MOVW only when you know the addend won't overflow? I suspect that for an overflow to occur it would require more than one global in the same section and each one accessed via a common base pointer + offset. For what its worth ARM's proprietary compiler used a separate RELA relocation section for MOVT/MOVW so that the addend could be encoded in the relocation; I don't think that this will work for llvm as not all linker's can cope with RELA relocations for ARM.

To the best of my knowledge the code and tests looks fine, but it is best if someone more familiar with the area gives it a check.


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