[LLVMdev] RFC: Constant Hoisting

[Jim Grosbach]

> On Feb 3, 2015, at 10:20 AM, Chris Sears <chris.sears at gmail.com> wrote:
> 
> I was using this GCC compiler comparison site. It's well done but unfortunately it's not up to date. The author Matt Godbolt said he was updating it:
> 
> http://gcc.godbolt.org/# <http://gcc.godbolt.org/#>
> 
> No, I do not have 3.4.1 but 3.5 is the current XCode release. It's pretty much already narrowed.

The Xcode release is from an Apple branch, which is not done at the same time as the LLVM 3.5 branch. Note the “svn” on the end of the 3.5 reference in the version info. That means that the Apple branch was done sometime between when open source 3.4 was branched and when open source 3.5 was branched.

-Jim

> 
> 3.4.1 generates correct assembly (except bug #2 is here as well, see below)
> 3.5 (XCode) generates correct IR and bad assembly
> 
> So the problem seems to show up with 3.5. The attached IR is from 3.5 XCode clang -S -O3 -emit-llvm newtst.c.
> 
> There are TWO bugs here. There's the Constant Hoisting bug (#1) and the failure to match a AND/CMP (#2). My current theory for #1 is that Constant Hoisting for the X86 is relatively recent or that Costing changed and that what we're seeing is that it worked correctly before. I think my proposed patch is correct and I'd recommend the ARM folks look at it.
> 
> I have no theory for bug #2 and I've already spent a day on it.
> <newtst.ll>_______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150203/4b054661/attachment.html>