[PATCH] D62990: [SCEV]When safe, use Stride as MinStart in computeMaxBECountForLT
Z. Zheng via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Thu Jun 20 12:12:28 PDT 2019
zzheng updated this revision to Diff 205871.
Repository:
rL LLVM
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D62990/new/
https://reviews.llvm.org/D62990
Files:
include/llvm/Analysis/ScalarEvolution.h
lib/Analysis/ScalarEvolution.cpp
test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
Index: test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
===================================================================
--- test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
+++ test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
@@ -1,7 +1,7 @@
; RUN: opt < %s -analyze -scalar-evolution 2>&1 | FileCheck %s
; CHECK: Loop %bb: backedge-taken count is ((999 + (-1 * %x)) /u 3)
-; CHECK: Loop %bb: max backedge-taken count is 334
+; CHECK: Loop %bb: max backedge-taken count is 333
; This is a tricky testcase for unsigned wrap detection which ScalarEvolution
Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -10465,8 +10465,8 @@
const SCEV *Stride,
const SCEV *End,
unsigned BitWidth,
- bool IsSigned) {
-
+ bool IsSigned,
+ const Loop *L) {
assert(!isKnownNonPositive(Stride) &&
"Stride is expected strictly positive!");
// Calculate the maximum backedge count based on the range of values
@@ -10475,6 +10475,34 @@
APInt MinStart =
IsSigned ? getSignedRangeMin(Start) : getUnsignedRangeMin(Start);
+ // For this loop:
+ // entry:
+ // %x = load i32, i32* @global, align 4
+ // %0 = icmp ult i32 %x, 17
+ // br i1 %0, label %loop, label %exit
+ //
+ // loop:
+ // %iv = phi i32 [ %x, %entry ], [ %iv.next, %loop ]
+ // %iv.next = add nuw i32 %iv, 8
+ // %1 = icmp ult i32 %iv.next, 17
+ // br i1 %1, label %loop, label %exit
+ //
+ // exit:
+ // ret void
+ //
+ // Start is (%x + 8), Stride is 8, End is 17. Given that loop entry is guarded
+ // by ult %x, 17, we know %x is in [0, 17) if loop is entered, and min start
+ // shall be 8.
+ auto *S = getMinusSCEV(Start, Stride);
+ if (!IsSigned && isa<SCEVConstant>(Stride) && isLoopInvariant(End, L) &&
+ // Prove that (Start - Stride) is in [0, End)
+ isLoopEntryGuardedByCond(L, ICmpInst::ICMP_ULT, S, End) &&
+ // Prove that (Start - Stride) doesn't overflow.
+ isKnownNonNegative(Stride) && isKnownNonNegative(End))
+ // Compute MinStart as getUnsignedRangeMin(Start-Stride) + Stride.
+ MinStart = getUnsignedRangeMin(S) +
+ dyn_cast<SCEVConstant>(Stride)->getAPInt();
+
APInt StrideForMaxBECount =
IsSigned ? getSignedRangeMin(Stride) : getUnsignedRangeMin(Stride);
@@ -10598,7 +10626,7 @@
// checked above).
if (!isLoopInvariant(RHS, L)) {
const SCEV *MaxBECount = computeMaxBECountForLT(
- Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
+ Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned, L);
return ExitLimit(getCouldNotCompute() /* ExactNotTaken */, MaxBECount,
false /*MaxOrZero*/, Predicates);
}
@@ -10635,7 +10663,7 @@
MaxOrZero = true;
} else {
MaxBECount = computeMaxBECountForLT(
- Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
+ Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned, L);
}
if (isa<SCEVCouldNotCompute>(MaxBECount) &&
Index: include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- include/llvm/Analysis/ScalarEvolution.h
+++ include/llvm/Analysis/ScalarEvolution.h
@@ -1838,7 +1838,7 @@
/// assert these preconditions so please be careful.
const SCEV *computeMaxBECountForLT(const SCEV *Start, const SCEV *Stride,
const SCEV *End, unsigned BitWidth,
- bool IsSigned);
+ bool IsSigned, const Loop *L);
/// Verify if an linear IV with positive stride can overflow when in a
/// less-than comparison, knowing the invariant term of the comparison,
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