[LLVMdev] ARM Jump table pcrelative relaxation in clang (original) (raw)

Eric Bentura ebentura at gmail.com
Mon Jul 13 05:41:05 PDT 2015

Hi, I have kept working on this and found the following (as llvm 3.5):

  1. In the function MCObjectStreamer::EmitInstruction there is a check for the instruction being relaxable or not: if (!Assembler.getBackend().mayNeedRelaxation(Inst)) { EmitInstToData(Inst, STI); return; } At this stage, the instruction as been already selected to be ARM::ADR. The call to mayNeed

2015-07-07 18:06 GMT+03:00 Tim Northover <t.p.northover at gmail.com>:

On 7 July 2015 at 06:06, Eric Bentura <ebentura at gmail.com> wrote: > I have created a small ll file to reproduce the problem. > I used the intrinsic function llvm.arm.space to introduce space between the > beginning of the code and the jump table.

It does look like the value in @llvm.arm.space is interpreted incorrectly if it's bigger than INTMAX, but that's well outside its intended range and could inevitably be used to break ConstantIslands (the longest ARM immediate branch is 26-bits; no indivisible entity in .text can be bigger than that). It's probably an unrelated issue. Also, I know I said the backend should accept any size code, but 2GB is definitely going to trigger more edge cases than average. Cheers. Tim. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150713/602e32d9/attachment.html>

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