[PATCH] D12886: Clean up: Refactoring the hardcoded value of 6 for FindAvailableLoadedValue()'s parameter MaxInstsToScan.

[Larisse Voufo via llvm-commits]

lvoufo created this revision.
lvoufo added reviewers: dberlin, nlewycky, resistor.
lvoufo added a subscriber: llvm-commits.

Any idea why this is hardcoded?
Is it okay to switch the value from 6 to 8 (or else) to resolve the following problem?

======================================================

Right now, in -O2, -early-cse replaces a load with a store, too early for -instcombine to recognize 
subsequent duplicate loads as actual duplicates to be removed.

For example, consider the following code snippet:
---------------------------------------------------------------------------------------------------
  %i = alloca %struct.A, align 4
  %j = alloca %struct.A, align 4
  %0 = bitcast %struct.A* %i to i8*
  call void @llvm.lifetime.start(i64 4, i8* %0) #3
  %call = call i32 @_Z3onev()
  %a2.i = getelementptr inbounds %struct.A, %struct.A* %i, i64 0, i32 0
  store i32 %call, i32* %a2.i, align 4, !tbaa !1
  %1 = bitcast %struct.A* %i to i8*
  %2 = call {}* @llvm.invariant.start(i64 4, i8* %1) #3
  %3 = bitcast %struct.A* %j to i8*
  call void @llvm.lifetime.start(i64 4, i8* %3) #3
  %4 = getelementptr inbounds %struct.A, %struct.A* %i, i64 0, i32 0
  %5 = getelementptr inbounds %struct.A, %struct.A* %j, i64 0, i32 0
  %6 = load i32, i32* %4, align 4
  store i32 %6, i32* %5, align 4, !tbaa !6
  %7 = bitcast %struct.A* %j to i8*
  %8 = call {}* @llvm.invariant.start(i64 4, i8* %7) #3
  call void @_Z3bar1A(i32 %6)
  call void @_Z3bar1A(i32 %6)
  call void @_Z4foo2PK1AS1_(%struct.A* nonnull %j, %struct.A* nonnull %i)
  %9 = getelementptr inbounds %struct.A, %struct.A* %i, i64 0, i32 0
  %10 = load i32, i32* %9, align 4       ;  <--- duplicate. Should be removed.
  call void @_Z3bar1A(i32 %10)
---------------------------------------------------------------------------------------------------

After -early-cse, the above becomes
---------------------------------------------------------------------------------------------------
  %i = alloca %struct.A, align 4
  %j = alloca %struct.A, align 4
  %0 = bitcast %struct.A* %i to i8*
  call void @llvm.lifetime.start(i64 4, i8* %0) #3
  %call = call i32 @_Z3onev()
  %a2.i = getelementptr inbounds %struct.A, %struct.A* %i, i64 0, i32 0
  store i32 %call, i32* %a2.i, align 4, !tbaa !1
  %1 = call {}* @llvm.invariant.start(i64 4, i8* %0) #3
  %2 = bitcast %struct.A* %j to i8*
  call void @llvm.lifetime.start(i64 4, i8* %2) #3
  %3 = getelementptr inbounds %struct.A, %struct.A* %j, i64 0, i32 0
  store i32 %call, i32* %3, align 4, !tbaa !6
  %4 = call {}* @llvm.invariant.start(i64 4, i8* %2) #3
  call void @_Z3bar1A(i32 %call)
  call void @_Z3bar1A(i32 %call)
  call void @_Z4foo2PK1AS1_(%struct.A* nonnull %j, %struct.A* nonnull %i)
  %5 = load i32, i32* %a2.i, align 4       ;  <--- duplicate. Should be removed.
  call void @_Z3bar1A(i32 %5)
---------------------------------------------------------------------------------------------------
where the first load from %i has been replaced with the store into %a2.i which points to %i.

For -instcombine to remove the duplicate load above, either
* -early-cse should not merge the first load into the store -- thereby treating the load as not a "trivially redundant instruction", or
* -instcombine should allow FindAvailableLoadedValue() to scan more than 6 instructions.
Note that @llvm.invariant.start() calls are ignored in the count, just like @llvm.lifetime.start().

A quick run has found the value of 8 to be the minimal needed, for this example case scenario.

======================================================


http://reviews.llvm.org/D12886

Files:
  include/llvm/Analysis/Loads.h
  lib/Analysis/Lint.cpp
  lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
  lib/Transforms/Scalar/JumpThreading.cpp

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