[llvm-dev] Optimization of successive constant stores

[Hal Finkel via llvm-dev]

Hi David,

We generally handle this (early) in the backend where we have more information about target capabilities and costs. See MergeConsecutiveStores in lib/CodeGen/SelectionDAG/DAGCombiner.cpp

 -Hal

----- Original Message -----
> From: "David Jones via llvm-dev" <llvm-dev at lists.llvm.org>
> To: llvm-dev at lists.llvm.org
> Sent: Friday, December 11, 2015 10:32:50 AM
> Subject: [llvm-dev] Optimization of successive constant stores
> 
> Consider the following:
> 
> t arget datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
> 
> %UodStructType = type { i8, i8, i8, i8, i32, i8* }
> 
> define void @test(%UodStructType*) {
> %2 = getelementptr inbounds %UodStructType* %0, i32 0, i32 0
> store i8 1, i8* %2, align 8
> %3 = getelementptr inbounds %UodStructType* %0, i32 0, i32 1
> store i8 2, i8* %3, align 1
> %4 = getelementptr inbounds %UodStructType* %0, i32 0, i32 2
> store i8 3, i8* %4, align 2
> %5 = getelementptr inbounds %UodStructType* %0, i32 0, i32 3
> store i8 4, i8* %5, align 1
> ret void
> }
> 
> If I run this through opt -O3, it passes through unchanged.
> 
> However, I would think that it would be profitable to combine the
> stores into a single instruction, e.g.:
> 
> define void @test(%UodStructType*) {
> %2 = bitcast %UodStructType* %0 to i32*
> store i32 0x04030201, i32* %2, align 8
> 
> 
> ret void
> }
> 
> 
> I don't see any optimization that would do this.
> 
> Interestingly, if I store the same 8-bit constant in all four bytes,
> then MemCpyOpt will indeed convert this to a 32-bit store.
> 
> 
> Am I doing something wrong, or is there really no optimization pass
> that can clean this up?
> 
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory