From 58ea4eeb9e812d7d9828f757ebfe7e574f92560b Mon Sep 17 00:00:00 2001 From: Jakub Kuderski Date: Thu, 10 Sep 2015 11:31:20 +0000 Subject: There is a trunc(lshr (zext A), Cst) optimization in InstCombineCasts that removes cast by performing the lshr on smaller types. However, currently there is no trunc(lshr (sext A), Cst) variant. This patch add such optimization by transforming trunc(lshr (sext A), Cst) to ashr A, Cst. Differential Revision: http://reviews.llvm.org/D12520 llvm-svn: 247271 --- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) (limited to 'llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp') diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp index 768616f..89fd3de0 100644 --- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp +++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp @@ -498,6 +498,26 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) { return CastInst::CreateIntegerCast(Shift, CI.getType(), false); } + // Transform trunc(lshr (sext A), Cst) to ashr A, Cst to eliminate type + // conversion. + // It works because bits coming from sign extension have the same value as + // sign bit of the original value; performing ashr instead of lshr + // generates bits of the same value as the sign bit. + if (Src->hasOneUse() && + match(Src, m_LShr(m_SExt(m_Value(A)), m_ConstantInt(Cst))) && + cast(Src)->getOperand(0)->hasOneUse()) { + const unsigned ASize = A->getType()->getPrimitiveSizeInBits(); + // This optimization can be only performed when zero bits generated by + // the original lshr aren't pulled into the value after truncation, so we + // can only shift by values smaller then the size of destination type (in + // bits). + if (Cst->getValue().ult(ASize)) { + Value *Shift = Builder->CreateAShr(A, Cst->getZExtValue()); + Shift->takeName(Src); + return CastInst::CreateIntegerCast(Shift, CI.getType(), true); + } + } + // Transform "trunc (and X, cst)" -> "and (trunc X), cst" so long as the dest // type isn't non-native. if (Src->hasOneUse() && isa(Src->getType()) && -- cgit v1.1