[llvm-dev] [GVN] same sequence of instructions in if and else branch

Tue Feb 9 12:51:42 PST 2016

Thanks all for replies.

Does LLVM  have a separate VBE pass? I couldn't find one. Although VBE is primarily for a code size, I see a case that VBE can affect following optimizations to reduce redundant moves in the binary.

Daniel, great to hear that you are working on a more advanced GVN implementation. I wonder if you have a timeline to upstream yours.

Best,
Taewook

From: Daniel Berlin <dberlin at dberlin.org<mailto:dberlin at dberlin.org>>
Date: Tuesday, February 9, 2016 at 12:28 PM
To: Taewook Oh <twoh at fb.com<mailto:twoh at fb.com>>
Cc: mats petersson <mats at planetcatfish.com<mailto:mats at planetcatfish.com>>, "llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] [GVN] same sequence of instructions in if and else branch

and by "right thing" i mean it can hoist if you want and it can prove it will not extend the live range.

Note that VBE (very busy expressions) is a code size optimization only. It does not save time.

On Tue, Feb 9, 2016 at 12:26 PM, Daniel Berlin <dberlin at dberlin.org<mailto:dberlin at dberlin.org>> wrote:
This GVN does not do that, this is correct. It is a very simple GVN. All phi nodes are considered to have unique values.
GCC's GVN, and the one i'm developing for LLVM at https://github.com/dberlin/llvm-gvn-rewrite  does the right thing.

On Tue, Feb 9, 2016 at 11:42 AM, Taewook Oh via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
There is a phi-node "%phi = phi i64 [%cast1, %if], [%cast2, %else]" in the common successor. The actual control flow is a bit more complex, but there is a common successor block, and %cast1 and %cast2 are the two values that the phi node in the common successor takes.

Does the existence of the phi node affect optimization?

Thanks,
Taewook

From: <mats.o.petersson at googlemail.com<mailto:mats.o.petersson at googlemail.com>> on behalf of mats petersson <mats at planetcatfish.com<mailto:mats at planetcatfish.com>>
Date: Tuesday, February 9, 2016 at 11:34 AM
To: Taewook Oh <twoh at fb.com<mailto:twoh at fb.com>>
Cc: "llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] [GVN] same sequence of instructions in if and else branch

And there's no PHI node after this that depends on the condition?

--
Mats

On 9 February 2016 at 19:30, Taewook Oh via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hello,

I found that GVN doesn't promote identical sequence of instructions in if and else branch to their common predecessors. For example, for the following code snippet

pred:

…

br i1 %cmp, label %if, label %else

if:

%incdec.ptr.1 = getelementptr inbounds i8, i8* %ptr, i64 1

%cast1 = ptrtoint i8* %incdec.ptr.1 to i64

    …

else:

%incdec.ptr.2 = getelementptr inbounds i8, i8* %ptr, i64 1

%cast2 = ptrtoint i8* %incdec.ptr.2 to i64

GVN doesn't move instructions in 'if' and 'else' blocks to 'pred' block even though it knows that incdec.ptr.1/case1 has a same value number with incdec.ptr.2/cast2. I see a case where this kind of redundancy confuses following optimization passes and ends up generating suboptimal code.

>From the GVN implementation, it seems that transformation is performed only when the "leader" value dominates the other value, so it cannot handle a case like the above example. I wonder if this is by design or just a missing feature.

Thanks,

Taewook

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