From 9419cab4c360d532bb5c3e171cdc8eca400b1b53 Mon Sep 17 00:00:00 2001 From: Owen Anderson Date: Thu, 19 Aug 2010 22:15:40 +0000 Subject: [PATCH] When a set of bitmask operations, typically from a bitfield initialization, only modifies the low bytes of a value, we can narrow the store to only over-write the affected bytes. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111568 91177308-0d34-0410-b5e6-96231b3b80d8 --- .../InstCombineLoadStoreAlloca.cpp | 45 +++++++++++++++++++ .../InstCombine/2010-08-19-StoreNarrowing.ll | 21 +++++++++ 2 files changed, 66 insertions(+) create mode 100644 test/Transforms/InstCombine/2010-08-19-StoreNarrowing.ll diff --git a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp index b68fbc2db5c..2f3d8b2b3e3 100644 --- a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp +++ b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp @@ -14,11 +14,13 @@ #include "InstCombine.h" #include "llvm/IntrinsicInst.h" #include "llvm/Analysis/Loads.h" +#include "llvm/Support/PatternMatch.h" #include "llvm/Target/TargetData.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/Transforms/Utils/Local.h" #include "llvm/ADT/Statistic.h" using namespace llvm; +using namespace PatternMatch; STATISTIC(NumDeadStore, "Number of dead stores eliminated"); @@ -473,6 +475,49 @@ Instruction *InstCombiner::visitStoreInst(StoreInst &SI) { if (SI.isVolatile()) return 0; // Don't hack volatile stores. + // Attempt to narrow sequences where we load a wide value, perform bitmasks + // that only affect the low bits of it, and then store it back. This + // typically arises from bitfield initializers in C++. + ConstantInt *CI1 =0, *CI2 = 0; + Value *Ld = 0; + if (getTargetData() && + match(SI.getValueOperand(), + m_And(m_Or(m_Value(Ld), m_ConstantInt(CI1)), m_ConstantInt(CI2))) && + isa(Ld) && + equivalentAddressValues(cast(Ld)->getPointerOperand(), Ptr)) { + APInt OrMask = CI1->getValue(); + APInt AndMask = CI2->getValue(); + + // Compute the prefix of the value that is unmodified by the bitmasking. + unsigned LeadingAndOnes = AndMask.countLeadingOnes(); + unsigned LeadingOrZeros = OrMask.countLeadingZeros(); + unsigned Prefix = std::min(LeadingAndOnes, LeadingOrZeros); + uint64_t NewWidth = AndMask.getBitWidth() - Prefix; + if (!isPowerOf2_64(NewWidth)) NewWidth = NextPowerOf2(NewWidth); + + // If we can find a power-of-2 prefix (and if the values we're working with + // are themselves POT widths), then we can narrow the store. We rely on + // later iterations of instcombine to propagate the demanded bits to narrow + // the other computations in the chain. + if (NewWidth < AndMask.getBitWidth() && + isPowerOf2_64(AndMask.getBitWidth())) { + const Type *NewType = IntegerType::get(Ptr->getContext(), NewWidth); + const Type *NewPtrType = PointerType::getUnqual(NewType); + + Value *NewVal = Builder->CreateTrunc(SI.getValueOperand(), NewType); + Value *NewPtr = Builder->CreateBitCast(Ptr, NewPtrType); + + // On big endian targets, we need to offset from the original pointer + // in order to store to the low-bit suffix. + if (getTargetData()->isBigEndian()) { + uint64_t GEPOffset = (AndMask.getBitWidth() - NewWidth) / 8; + NewPtr = Builder->CreateConstGEP1_64(NewPtr, GEPOffset); + } + + return new StoreInst(NewVal, NewPtr); + } + } + // store X, null -> turns into 'unreachable' in SimplifyCFG if (isa(Ptr) && SI.getPointerAddressSpace() == 0) { if (!isa(Val)) { diff --git a/test/Transforms/InstCombine/2010-08-19-StoreNarrowing.ll b/test/Transforms/InstCombine/2010-08-19-StoreNarrowing.ll new file mode 100644 index 00000000000..d4d528943fe --- /dev/null +++ b/test/Transforms/InstCombine/2010-08-19-StoreNarrowing.ll @@ -0,0 +1,21 @@ +; RUN: opt -S -instcombine %s | not grep and +target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64" +target triple = "x86_64-apple-darwin10.0.0" + +%class.A = type { i8, [3 x i8] } + +define void @_ZN1AC2Ev(%class.A* %this) nounwind ssp align 2 { +entry: + %0 = bitcast %class.A* %this to i32* ; [#uses=5] + %1 = load i32* %0, align 4 ; [#uses=1] + %2 = and i32 %1, -8 ; [#uses=2] + store i32 %2, i32* %0, align 4 + %3 = and i32 %2, -57 ; [#uses=1] + %4 = or i32 %3, 8 ; [#uses=2] + store i32 %4, i32* %0, align 4 + %5 = and i32 %4, -65 ; [#uses=2] + store i32 %5, i32* %0, align 4 + %6 = and i32 %5, -129 ; [#uses=1] + store i32 %6, i32* %0, align 4 + ret void +} -- 2.34.1