[ScopDetect] Reject loop with multiple exit blocks.

The current statement domain derivation algorithm does not (always) consider that different exit blocks of a loop can have different conditions to be reached. From the code for (int i = n; ; i-=2) { if (i <= 0) goto even; if (i <= 1) goto odd; A[i] = i; } even: A[0] = 42; return; odd: A[1] = 21; return; Polly currently derives the following domains: Stmt_even_critedge Domain := [n] -> { Stmt_even_critedge[] }; Stmt_odd Domain := [n] -> { Stmt_odd[] : (1 + n) mod 2 = 0 and n > 0 }; while the domain for the odd case is correct, Stmt_even is assumed to be executed unconditionally, which is obviously wrong. While projecting out the loop dimension in `adjustDomainDimensions`, it does not consider that there are other exit condition that have matched before. I don't know a how to fix this without changing a lot of code. Therefore This patch rejects loops with multiple exist blocks to fix the miscompile of test-suite's uuencode. The odd condition is transformed by LLVM to %cmp1 = icmp eq i64 %indvars.iv, 1 such that the project_out in adjustDomainDimensions() indeed only matches for odd n (using this condition only, we'd have an infinite loop otherwise). The even condition manifests as %cmp = icmp slt i64 %indvars.iv, 3 Because buildDomainsWithBranchConstraints() does not consider other exit conditions, it has to assume that the induction variable will eventually be lower than 3 and taking this exit. IMHO we need to reuse the algorithm that determines the number of iterations (addLoopBoundsToHeaderDomain) to determine which exit condition applies first. It has to happen in buildDomainsWithBranchConstraints() because the result will need to propagate to successor BBs. Currently addLoopBoundsToHeaderDomain() just look for union of all backedge conditions (which means leaving not the loop here). The patch in llvm.org/PR35465 changes it to look for exit conditions instead. This is required because there might be other exit conditions that do not alternatively go back to the loop header. Differential Revision: https://reviews.llvm.org/D45649 llvm-svn: 330858
llvm · Apr 25, 2018 · beffdb9 · beffdb9
1 parent d8803d3
commit beffdb9
Showing 15 changed files with 520 additions and 1 deletion.
diff --git a/polly/include/polly/ScopDetectionDiagnostic.h b/polly/include/polly/ScopDetectionDiagnostic.h
@@ -90,6 +90,7 @@ enum class RejectReasonKind {
 
   LoopBound,
   LoopHasNoExit,
+  LoopHasMultipleExits,
   LoopOnlySomeLatches,
 
   FuncCall,
@@ -584,6 +585,34 @@ class ReportLoopHasNoExit : public RejectReason {
   //@}
 };
 
+//===----------------------------------------------------------------------===//
+/// Captures errors when a loop has multiple exists.
+class ReportLoopHasMultipleExits : public RejectReason {
+  /// The loop that has multiple exits.
+  Loop *L;
+
+  const DebugLoc Loc;
+
+public:
+  ReportLoopHasMultipleExits(Loop *L)
+      : RejectReason(RejectReasonKind::LoopHasMultipleExits), L(L),
+        Loc(L->getStartLoc()) {}
+
+  /// @name LLVM-RTTI interface
+  //@{
+  static bool classof(const RejectReason *RR);
+  //@}
+
+  /// @name RejectReason interface
+  //@{
+  std::string getRemarkName() const override;
+  const Value *getRemarkBB() const override;
+  std::string getMessage() const override;
+  const DebugLoc &getDebugLoc() const override;
+  std::string getEndUserMessage() const override;
+  //@}
+};
+
 //===----------------------------------------------------------------------===//
 /// Captures errors when not all loop latches are part of the scop.
 class ReportLoopOnlySomeLatches : public RejectReason {

diff --git a/polly/lib/Analysis/ScopDetection.cpp b/polly/lib/Analysis/ScopDetection.cpp
@@ -1297,6 +1297,18 @@ bool ScopDetection::isValidLoop(Loop *L, DetectionContext &Context) const {
   if (!hasExitingBlocks(L))
     return invalid<ReportLoopHasNoExit>(Context, /*Assert=*/true, L);
 
+  // The algorithm for domain construction assumes that loops has only a single
+  // exit block (and hence corresponds to a subregion). Note that we cannot use
+  // L->getExitBlock() because it does not check whether all exiting edges point
+  // to the same BB.
+  SmallVector<BasicBlock *, 4> ExitBlocks;
+  L->getExitBlocks(ExitBlocks);
+  BasicBlock *TheExitBlock = ExitBlocks[0];
+  for (BasicBlock *ExitBB : ExitBlocks) {
+    if (TheExitBlock != ExitBB)
+      return invalid<ReportLoopHasMultipleExits>(Context, /*Assert=*/true, L);
+  }
+
   if (canUseISLTripCount(L, Context))
     return true;
 

diff --git a/polly/lib/Analysis/ScopDetectionDiagnostic.cpp b/polly/lib/Analysis/ScopDetectionDiagnostic.cpp
@@ -71,6 +71,7 @@ Statistic RejectStatistics[] = {
     SCOP_STAT(LastAffFunc, ""),
     SCOP_STAT(LoopBound, "Uncomputable loop bounds"),
     SCOP_STAT(LoopHasNoExit, "Loop without exit"),
+    SCOP_STAT(LoopHasMultipleExits, "Loop with multiple exits"),
     SCOP_STAT(LoopOnlySomeLatches, "Not all loop latches in scop"),
     SCOP_STAT(FuncCall, "Function call with side effects"),
     SCOP_STAT(NonSimpleMemoryAccess,
@@ -495,6 +496,31 @@ std::string ReportLoopHasNoExit::getEndUserMessage() const {
   return "Loop cannot be handled because it has no exit.";
 }
 
+//===----------------------------------------------------------------------===//
+// ReportLoopHasMultipleExits.
+
+std::string ReportLoopHasMultipleExits::getRemarkName() const {
+  return "ReportLoopHasMultipleExits";
+}
+
+const Value *ReportLoopHasMultipleExits::getRemarkBB() const {
+  return L->getHeader();
+}
+
+std::string ReportLoopHasMultipleExits::getMessage() const {
+  return "Loop " + L->getHeader()->getName() + " has multiple exits.";
+}
+
+bool ReportLoopHasMultipleExits::classof(const RejectReason *RR) {
+  return RR->getKind() == RejectReasonKind::LoopHasMultipleExits;
+}
+
+const DebugLoc &ReportLoopHasMultipleExits::getDebugLoc() const { return Loc; }
+
+std::string ReportLoopHasMultipleExits::getEndUserMessage() const {
+  return "Loop cannot be handled because it has multiple exits.";
+}
+
 //===----------------------------------------------------------------------===//
 // ReportLoopOnlySomeLatches
 

diff --git a/polly/test/ScopDetect/index_from_unpredictable_loop.ll b/polly/test/ScopDetect/index_from_unpredictable_loop.ll
@@ -1,6 +1,12 @@
 ; RUN: opt %loadPolly -polly-scops -analyze < %s | FileCheck %s --check-prefix=AFFINE
 ; RUN: opt %loadPolly -polly-scops -polly-allow-nonaffine -analyze < %s | FileCheck %s --check-prefix=NONAFFINE
 
+; The SCoP contains a loop with multiple exit blocks (BBs after leaving
+; the loop). The current implementation of deriving their domain derives
+; only a common domain for all of the exit blocks. We disabled loops with
+; multiple exit blocks until this is fixed.
+; XFAIL: *
+
 ; The loop for.body => for.inc has an unpredictable iteration count could due to
 ; the undef start value that it is compared to. Therefore the array element
 ; %arrayidx101 that depends on that exit value cannot be affine.

diff --git a/polly/test/ScopDetect/index_from_unpredictable_loop2.ll b/polly/test/ScopDetect/index_from_unpredictable_loop2.ll
@@ -1,6 +1,12 @@
 ; RUN: opt %loadPolly -polly-scops -analyze < %s | FileCheck %s --check-prefix=AFFINE
 ; RUN: opt %loadPolly -polly-scops -polly-allow-nonaffine -analyze < %s | FileCheck %s --check-prefix=NONAFFINE
 
+; The SCoP contains a loop with multiple exit blocks (BBs after leaving
+; the loop). The current implementation of deriving their domain derives
+; only a common domain for all of the exit blocks. We disabled loops with
+; multiple exit blocks until this is fixed.
+; XFAIL: *
+
 ; The loop for.body => for.inc has an unpredictable iteration count could due to
 ; the undef start value that it is compared to. Therefore the array element
 ; %arrayidx101 that depends on that exit value cannot be affine.