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[SCEV] Strengthen SCEVExpander::isHighCostExpansion. 

Summary:

Teach `isHighCostExpansion` to consider divisions by power-of-two
constants as cheap and add a test case.  This change is needed for a new
user of `isHighCostExpansion` that will be added in a subsequent change.

Depends on D8995.

Reviewers: atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8993

llvm-svn: 234845
This commit is contained in:
Sanjoy Das 2015-04-14 03:20:32 +00:00
parent 2e6bb3b947
commit a9f1e27a04
2 changed files with 46 additions and 6 deletions
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23
llvm/lib/Analysis/ScalarEvolutionExpander.cpp
View File

@ -1810,12 +1810,23 @@ bool SCEVExpander::isHighCostExpansionHelper(
if (!Processed.insert(S).second)
return false;
// If the backedge-taken count is a UDiv, it's very likely a UDiv that
// ScalarEvolution's HowFarToZero or HowManyLessThans produced to compute a
// precise expression, rather than a UDiv from the user's code. If we can't
// find a UDiv in the code with some simple searching, assume the former and
// forego rewriting the loop.
if (isa<SCEVUDivExpr>(S)) {
if (auto *UDivExpr = dyn_cast<SCEVUDivExpr>(S)) {
// If the divisor is a power of two and the SCEV type fits in a native
// integer, consider the divison cheap irrespective of whether it occurs in
// the user code since it can be lowered into a right shift.
if (auto *SC = dyn_cast<SCEVConstant>(UDivExpr->getRHS()))
if (SC->getValue()->getValue().isPowerOf2()) {
const DataLayout &DL =
L->getHeader()->getParent()->getParent()->getDataLayout();
unsigned Width = cast<IntegerType>(UDivExpr->getType())->getBitWidth();
return DL.isIllegalInteger(Width);
}
// UDivExpr is very likely a UDiv that ScalarEvolution's HowFarToZero or
// HowManyLessThans produced to compute a precise expression, rather than a
// UDiv from the user's code. If we can't find a UDiv in the code with some
// simple searching, assume the former consider UDivExpr expensive to
// compute.
BasicBlock *ExitingBB = L->getExitingBlock();
if (!ExitingBB)
return true;

29
llvm/test/Transforms/IndVarSimplify/lftr-udiv-tripcount.ll Normal file
View File

@ -0,0 +1,29 @@
; RUN: opt < %s -indvars -S | FileCheck %s
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-S128"
; It is okay to do LFTR on this loop even though the trip count is a
; division because in this case the division can be optimized to a
; shift.
define void @foo(i8* %a, i8 %n) nounwind uwtable ssp {
; CHECK-LABEL: @foo(
entry:
%e = icmp sgt i8 %n, 3
br i1 %e, label %loop, label %exit
loop:
; CHECK-LABEL: loop:
%i = phi i8 [ 0, %entry ], [ %i.inc, %loop ]
%i1 = phi i8 [ 0, %entry ], [ %i1.inc, %loop ]
%i.inc = add nsw i8 %i, 4
%i1.inc = add i8 %i1, 1
store volatile i8 0, i8* %a
%c = icmp slt i8 %i, %n
; CHECK-LABEL: %exitcond = icmp ne i8 %i1.inc
br i1 %c, label %loop, label %exit
exit:
; CHECK-LABEL: exit:
ret void
}