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[SCEV] Be more aggressive in proving NUW 

... for AddRec's in loops for which SCEV is unable to compute a max
tripcount.  This is the NUW variant of r269211 and fixes PR27691.

(Note: PR27691 is not a correct or stability bug, it was created to
track a pending task).

llvm-svn: 269790
This commit is contained in:
Sanjoy Das 2016-05-17 17:51:14 +00:00
parent bc6a7df072
commit f5d40d5350
2 changed files with 80 additions and 7 deletions
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27
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -1521,11 +1521,22 @@ const SCEV *ScalarEvolution::getZeroExtendExpr(const SCEV *Op,
getSignExtendExpr(Step, Ty), L, AR->getNoWrapFlags());
}
}
}
// If the backedge is guarded by a comparison with the pre-inc value
// the addrec is safe. Also, if the entry is guarded by a comparison
// with the start value and the backedge is guarded by a comparison
// with the post-inc value, the addrec is safe.
// Normally, in the cases we can prove no-overflow via a
// backedge guarding condition, we can also compute a backedge
// taken count for the loop. The exceptions are assumptions and
// guards present in the loop -- SCEV is not great at exploiting
// these to compute max backedge taken counts, but can still use
// these to prove lack of overflow. Use this fact to avoid
// doing extra work that may not pay off.
if (!isa<SCEVCouldNotCompute>(MaxBECount) || HasGuards ||
!AC.assumptions().empty()) {
// If the backedge is guarded by a comparison with the pre-inc
// value the addrec is safe. Also, if the entry is guarded by
// a comparison with the start value and the backedge is
// guarded by a comparison with the post-inc value, the addrec
// is safe.
if (isKnownPositive(Step)) {
const SCEV *N = getConstant(APInt::getMinValue(BitWidth) -
getUnsignedRange(Step).getUnsignedMax());
@ -1533,7 +1544,8 @@ const SCEV *ScalarEvolution::getZeroExtendExpr(const SCEV *Op,
(isLoopEntryGuardedByCond(L, ICmpInst::ICMP_ULT, Start, N) &&
isLoopBackedgeGuardedByCond(L, ICmpInst::ICMP_ULT,
AR->getPostIncExpr(*this), N))) {
// Cache knowledge of AR NUW, which is propagated to this AddRec.
// Cache knowledge of AR NUW, which is propagated to this
// AddRec.
const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNUW);
// Return the expression with the addrec on the outside.
return getAddRecExpr(
@ -1547,8 +1559,9 @@ const SCEV *ScalarEvolution::getZeroExtendExpr(const SCEV *Op,
(isLoopEntryGuardedByCond(L, ICmpInst::ICMP_UGT, Start, N) &&
isLoopBackedgeGuardedByCond(L, ICmpInst::ICMP_UGT,
AR->getPostIncExpr(*this), N))) {
// Cache knowledge of AR NW, which is propagated to this AddRec.
// Negative step causes unsigned wrap, but it still can't self-wrap.
// Cache knowledge of AR NW, which is propagated to this
// AddRec. Negative step causes unsigned wrap, but it
// still can't self-wrap.
const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNW);
// Return the expression with the addrec on the outside.
return getAddRecExpr(

60
llvm/test/Analysis/ScalarEvolution/no-wrap-unknown-becount.ll
View File

@ -62,3 +62,63 @@ loop:
leave:
ret void
}
define void @u_0(i32 %n, i1* %cond) {
; CHECK-LABEL: Classifying expressions for: @u_0
entry:
br label %loop
loop:
%iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
%iv.inc = add i32 %iv, 1
%iv.zext = zext i32 %iv to i64
; CHECK: %iv.zext = zext i32 %iv to i64
; CHECK-NEXT: --> {0,+,1}<nuw><%loop>
%cmp = icmp ult i32 %iv, %n
call void(i1, ...) @llvm.experimental.guard(i1 %cmp) [ "deopt"() ]
%c = load volatile i1, i1* %cond
br i1 %c, label %loop, label %leave
leave:
ret void
}
define void @u_1(i1* %cond) {
; CHECK-LABEL: Classifying expressions for: @u_1
entry:
br label %loop
loop:
%iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
%iv.inc = add i32 %iv, 3
%iv.zext = zext i32 %iv to i64
; CHECK: %iv.zext = zext i32 %iv to i64
; CHECK-NEXT: --> {0,+,3}<nuw><%loop>
%cmp = icmp ult i32 %iv, 10000
call void(i1, ...) @llvm.experimental.guard(i1 %cmp) [ "deopt"() ]
%c = load volatile i1, i1* %cond
br i1 %c, label %loop, label %leave
leave:
ret void
}
define void @u_2(i1* %cond) {
; CHECK-LABEL: Classifying expressions for: @u_2
entry:
br label %loop
loop:
%iv = phi i32 [ 30000, %entry ], [ %iv.inc, %loop ]
%iv.inc = add i32 %iv, -2
%iv.zext = zext i32 %iv to i64
%cmp = icmp ugt i32 %iv.inc, -10000
; CHECK: %iv.zext = zext i32 %iv to i64
; CHECK-NEXT: --> {30000,+,-2}<nw><%loop>
call void @llvm.assume(i1 %cmp)
%c = load volatile i1, i1* %cond
br i1 %c, label %loop, label %leave
leave:
ret void
}