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[SCEV] Reapply 'Teach isLoopBackedgeGuardedByCond to exploit trip counts' 

Summary:
If the trip count of a specific backedge is `N`, then we know that
backedge is effectively guarded by the condition `{0,+,1} u< N`.  This
change teaches SCEV to use this condition to prove things in
`isLoopBackedgeGuardedByCond`.

Depends on D12948
Depends on D12949

The original checkin, r248608 had to be backed out due to an issue with
a ObjCXX unit test.  That issue is now fixed, so re-landing.

Reviewers: atrick, reames, majnemer, hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12950

llvm-svn: 248638
This commit is contained in:
Sanjoy Das 2015-09-25 23:53:50 +00:00
parent 96709c4854
commit b174f9a316
3 changed files with 55 additions and 2 deletions
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16
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -6994,6 +6994,22 @@ ScalarEvolution::isLoopBackedgeGuardedByCond(const Loop *L,
SaveAndRestore<bool> ClearOnExit(WalkingBEDominatingConds, true);
// See if we can exploit a trip count to prove the predicate.
const auto &BETakenInfo = getBackedgeTakenInfo(L);
const SCEV *LatchBECount = BETakenInfo.getExact(Latch, this);
if (LatchBECount != getCouldNotCompute()) {
// We know that Latch branches back to the loop header exactly
// LatchBECount times. This means the backdege condition at Latch is
// equivalent to "{0,+,1} u< LatchBECount".
Type *Ty = LatchBECount->getType();
auto NoWrapFlags = SCEV::NoWrapFlags(SCEV::FlagNUW | SCEV::FlagNW);
const SCEV *LoopCounter =
getAddRecExpr(getZero(Ty), getOne(Ty), L, NoWrapFlags);
if (isImpliedCond(Pred, LHS, RHS, ICmpInst::ICMP_ULT, LoopCounter,
LatchBECount))
return true;
}
// Check conditions due to any @llvm.assume intrinsics.
for (auto &AssumeVH : AC.assumptions()) {
if (!AssumeVH)

2
llvm/test/Analysis/ScalarEvolution/zext-wrap.ll
View File

@ -10,7 +10,7 @@ bb.i: ; preds = %bb1.i, %bb.nph
; This cast shouldn't be folded into the addrec.
; CHECK: %tmp = zext i8 %l_95.0.i1 to i16
; CHECK: --> (zext i8 {0,+,-1}<%bb.i> to i16){{ U: [^ ]+ S: [^ ]+}}{{ *}}Exits: 2
; CHECK: --> (zext i8 {0,+,-1}<nw><%bb.i> to i16){{ U: [^ ]+ S: [^ ]+}}{{ *}}Exits: 2
%tmp = zext i8 %l_95.0.i1 to i16

39
llvm/test/Transforms/IndVarSimplify/eliminate-comparison.ll
View File

@ -356,5 +356,42 @@ define void @func_17(i32* %len.ptr) {
ret void
}
!0 = !{i32 0, i32 2147483647}
define i1 @func_18(i16* %tmp20, i32* %len.addr) {
; CHECK-LABEL: @func_18(
entry:
%len = load i32, i32* %len.addr, !range !0
%tmp18 = icmp eq i32 %len, 0
br i1 %tmp18, label %bb2, label %bb0.preheader
bb0.preheader:
br label %bb0
bb0:
; CHECK: bb0:
%var_0.in = phi i32 [ %var_0, %bb1 ], [ %len, %bb0.preheader ]
%var_1 = phi i32 [ %tmp30, %bb1 ], [ 0, %bb0.preheader ]
%var_0 = add nsw i32 %var_0.in, -1
%tmp23 = icmp ult i32 %var_1, %len
; CHECK: br i1 true, label %stay, label %bb2.loopexit
br i1 %tmp23, label %stay, label %bb2
stay:
; CHECK: stay:
%tmp25 = getelementptr inbounds i16, i16* %tmp20, i32 %var_1
%tmp26 = load i16, i16* %tmp25
%tmp29 = icmp eq i16 %tmp26, 0
br i1 %tmp29, label %bb1, label %bb2
bb1:
%tmp30 = add i32 %var_1, 1
%tmp31 = icmp eq i32 %var_0, 0
br i1 %tmp31, label %bb3, label %bb0
bb2:
ret i1 false
bb3:
ret i1 true
}
!0 = !{i32 0, i32 2147483647}