Invalid instructions (UMAAL) are generated for the thumbv7m-none-eabi target

[japaric]

UMAAL is only available in the ARMv7E-M processors but if you compile core for the thumbv7m (ARMv7-M) target, rustc/LLVM generates them for the core::num::flt2dec::strategy::grisu::format_shortest_opt function.

[japaric]

Relevant information: See the ARM Cortex-M instructions groups table under instructions sets. UMAAL is in the group of instructions that are not available for the Cortex-M3, which is the processor the thumbv7m target covers.

Building core with a explicit -C target-cpu=cortex-m3 still generates the UMAAL instruction. This seems like a bug in LLVM.

[pftbest]

Minimal IR that triggers the problem:
bugpoint-reduced-simplified.ll
It generates umaal instruction when is compiled by llc

[pftbest]

I have submitted the fix for review:
https://reviews.llvm.org/D25890

[pftbest]

Fixed in llvm upstream, r285278 and r285280

[pftbest]

How does llvm version gets updated? Do I need to backport this fix to llvm 3.9 somehow, so it could land in rust?

[alexcrichton]

@pftbest nice! To update LLVM there's two routes:

1. Backport the patches to our fork, currently around the 3.9 release.
2. Upgrade our fork to the current LLVM head

(1) is much easier than (2) typically, and we're fine with both!

[pftbest]

So, I've rebased rust-llvm-2016-07-09 branch to match llvm version 3.9.1 and then I backported my fix on top of it. The new branch is called rust-llvm-2016-10-29 and I don't know what to do next.

[japaric]

@pftbest I think:

• Locally, make the rust repo's llvm submodule point to your 10-29 branch and test (make check) locally.
• If that works, ask a core member someone with push access to the rust-lang/llvm repo to create a new 10-29 branch there
• Then send another PR to rust-lang/rust updating its llvm submodule to the new branch in rust-lang/llvm
• Cross your fingers and hope that it passes the buildbots.

[alexcrichton]

Ok I've pushed your branch to rust-lang/llvm (thanks for the rebase to 3.9.1!). As @japaric mentioned if you want to send a PR to rust-lang/rust updating the submodule I'll r+ that

[pftbest]

Thank you! I have submitted a #37465