[llvm-dev] Load combine pass

[Philip Reames via llvm-dev]

There's a bit of additional context worth adding here...

Up until very recently, we had a form of widening implemented in GVN.  
We decided to remove this in https://reviews.llvm.org/D24096 precisely 
because its placement in the pass pipeline was inhibiting other 
optimizations. There's also a major problem with doing widening at the 
IR level which is that widening a pair of atomic loads into a single 
wider atomic load can not be undone. This creates a major pass ordering 
problem of its own.

At this point, my general view is that widening transformations of any 
kind should be done very late.  Ideally, this is something the backend 
would do, but doing it as a CGP like fixup pass over the IR is also 
reasonable.

With that in mind, I feel both the current placement of LoadCombine 
(within the inliner iteration) and the proposed InstCombine rule are 
undesirable.

Philip


On 09/28/2016 08:22 AM, Artur Pilipenko wrote:
> Hi,
>
> I'm trying to optimize a pattern like this into a single i16 load:
>   %1 = bitcast i16* %pData to i8*
>   %2 = load i8, i8* %1, align 1
>   %3 = zext i8 %2 to i16
>   %4 = shl nuw i16 %3, 8
>   %5 = getelementptr inbounds i8, i8* %1, i16 1
>   %6 = load i8, i8* %5, align 1
>   %7 = zext i8 %6 to i16
>   %8 = shl nuw nsw i16 %7, 0
>   %9 = or i16 %8, %4
>
> I came across load combine pass which is motivated by virtualliy the 
> same pattern. Load combine optimizes the pattern by combining adjacent 
> loads into one load and lets the rest of the optimizer cleanup the 
> rest. From what I see on the initial review for load combine 
> (https://reviews.llvm.org/D3580) it was not enabled by default because 
> it caused some performance regressions. It's not very surprising, I 
> see how this type of widening can obscure some facts for the rest of 
> the optimizer.
>
> I can't find any backstory for this pass, why was it chosen to 
> optimize the pattern in question in this way? What is the current 
> status of this pass?
>
> I have an alternative implementation for it locally. I implemented an 
> instcombine rule similar to recognise bswap/bitreverse idiom. It 
> relies on collectBitParts (Local.cpp) to determine the origin of the 
> bits in a given or value. If all the bits are happen to be loaded from 
> adjacent locations it replaces the or with a single load or a load 
> plus bswap.
>
> If the alternative approach sounds reasonable I'll post my patches for 
> review.
>
> Artur