[llvm-dev] The state of ARMConstantIslandPass in 4.0.[01]

[Tom Stellard via llvm-dev]

On 05/29/2017 05:41 PM, Ariel Ben-Yehuda via llvm-dev wrote:
> Hi,
> 
> We at Rust are using LLVM for our codegen. Since Rust version 1.19.0,
> which should ship in July 21st, we are using a slightly patched
> version of LLVM 4.0.
> 
> The transition to LLVM 4.0 had been fairly pain-free on x86 (at least
> excluding the standard suite of assertion failures using MSVC SEH, but
> that's fairly under control), but we have encountered several scary
> bugs in ARM, most specifically in ARMConstantIslandPass.cpp:
> 
>  - https://github.com/rust-lang/rust/issues/41672
>  - https://github.com/rust-lang/rust/issues/42248
> 
> The former of these is a fairly benign assertion failure, but the
> latter is a scary "in the wild" wrong codegen bug, which can really
> mess up with programmers.
> 
> Both of the bugs have fixes in trunk:
> r295964 - Fix assertion failure in ARMConstantIslandPass.
> r297871 - ARM: avoid clobbering register in v6 jump-table expansion.
> 
> I can easily backport the fixes for these 2 bugs into our LLVM branch.
> However, while looking at the diff between 4.0 and trunk I've seen
> several more fixes to "scary-looking" bugs:
> 
> r294949 - [Thumb-1] TBB generation: spot redefinitions of index register
> r295816 - [ARM] Fix constant islands pass.
> r299634 - [ARM] Remove a dead ADD during the creation of TBBs
>     (that one looks more like an optimization than a wrong-codegen
> thing, but including for completeness)
> r300870 - [Thumb-1] Fix corner cases for compressed jump tables
> 
> After seeing the previous ones, I'm worried these other fixes might
> also fix some "in the wild" wrong codegen bugs. However, I'm not
> comfortable backporting so many patches by myself. We are open to
> moving to being based on LLVM 4.0.1 when it is released, but it does
> not seem to include *any* of the above fixes.
> 
> Are these fixes important? Should we backport them? Do we need to keep
> anything in mind?
> 

It's probably too late for these to make it into the 4.0.1 release,
since the merge deadline is today, but please submit merge requests
for these anyway (preferred way is to use the utils/release/merge-request.sh
script), because it will help us determine if we need a 4.0.2 release.

Thanks,
Tom

> Thanks in advance,
>     - Ariel
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