[PATCH] Constant fold llvm.expect(c, ...) -> c

[Chandler Carruth]

On Mon, Jan 26, 2015 at 11:29 AM, Pete Cooper <peter_cooper at apple.com>
wrote:

> That would be nice.  I expect (ha!) that we’d do this in IRGen.  I might
> look in to this, although i’m not too familiar with clang.
>
> If you’re interested, i’ve worked out why I see this benefit at all in
> this patch.  I LTO my Debug LLVM.  This results in -O0 on the bitcode
> produced by LLVM, but then the linker LTO’s with optimizations enabled.
> And because LTO doesn’t have LowerExpect in its pipeline, I see many cases
> of 'call i64 @llvm.expect.i64(i64 1, i64 1)’ in my IR.
>
> So, getting the front-end to generate metadata would certainly fix this.
> As would calling LowerExpect in libLTO or in populateFunctionPassManager
> even when on -O0.  Or we could just say that a Debug LTO build should live
> with this issue, although I’d prefer one of the other options to this one.
>

First problem: why are we generating these at -O0 at all? We should just
turn that part of Clang off.

Second problem: ... nope, no second problem. ;]
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