[Polly] We do not support alias checks for basepointers defined inside the SCoP

[grosser]

Summary

The run-time alias check places code that involves the base pointer at the
beginning of the SCoP. This breaks if the base pointer is defined inside the
scop. Hence, we can only use it if we are sure the base pointer is not an
instruction defined inside the scop. Otherwise we refuse to handle the SCoP.

This commit should unbreak some of our current LNT failures.

[grosser]

Remove dump statement

[jdoerfert]

LGTM

[grosser]

Closed by commit rL218412 (authored by @grosser).

polly/trunk/lib/Analysis/ScopDetection.cpp

if (!isAffineExpr(&Context.CurRegion, AccessFunction, *SE, BaseValue))
                                             AccessFunction, &Inst, BaseValue);
   }
 
   // FIXME: Alias Analysis thinks IntToPtrInst aliases with alloca instructions
   // created by IndependentBlocks Pass.
   if (IntToPtrInst *Inst = dyn_cast<IntToPtrInst>(BaseValue))
     return invalid<ReportIntToPtr>(Context, /*Assert=*/true, Inst);
 
-  if (PollyUseRuntimeAliasChecks || IgnoreAliasing)
+  if (IgnoreAliasing)
     return true;
 
   // Check if the base pointer of the memory access does alias with
   // any other pointer. This cannot be handled at the moment.
   AliasSet &AS =
       Context.AST.getAliasSetForPointer(BaseValue, AliasAnalysis::UnknownSize,
                                         Inst.getMetadata(LLVMContext::MD_tbaa));
 
   // INVALID triggers an assertion in verifying mode, if it detects that a
   // SCoP was detected by SCoP detection and that this SCoP was invalidated by
   // a pass that stated it would preserve the SCoPs. We disable this check as
   // the independent blocks pass may create memory references which seem to
   // alias, if -basicaa is not available. They actually do not, but as we can
   // not proof this without -basicaa we would fail. We disable this check to
   // not cause irrelevant verification failures.
-  if (!AS.isMustAlias())
+  if (!AS.isMustAlias()) {
+    if (PollyUseRuntimeAliasChecks) {
+      bool CanBuildRunTimeCheck = true;
+      // The run-time alias check places code that involves the base pointer at
+      // the beginning of the SCoP. This breaks if the base pointer is defined
+      // inside the scop. Hence, we can only create a run-time check if we are
+      // sure the base pointer is not an instruction defined inside the scop.
+      for (const auto &Ptr : AS) {
+        Instruction *Inst = dyn_cast<Instruction>(Ptr.getValue());
+        if (Inst && Context.CurRegion.contains(Inst)) {
+          CanBuildRunTimeCheck = false;
+          break;
+        }
+      }
+
+      if (CanBuildRunTimeCheck)
+        return true;
+    }
     return invalid<ReportAlias>(Context, /*Assert=*/false, &Inst, AS);
+  }
 
   return true;
 }
 
 bool ScopDetection::isValidInstruction(Instruction &Inst,
                                        DetectionContext &Context) const {
   if (PHINode *PN = dyn_cast<PHINode>(&Inst))
     if (!canSynthesize(PN, LI, SE, &Context.CurRegion)) {

polly/trunk/test/ScopDetect/run_time_alias_check.ll

+; RUN: opt %loadPolly -polly-detect -polly-code-generator=isl -analyze < %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"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare float* @getNextBasePtr(float*) readnone nounwind
+
+define void @base_pointer_is_inst_inside_invariant_1(i64 %n, float* %A, float* %B) {
+entry:
+  br label %for.i
+
+for.i:
+  %indvar.i = phi i64 [ %indvar.i.next, %for.i.inc ], [ 0, %entry ]
+  br label %S1
+
+S1:
+; To get an instruction inside a region, we use a function without side
+; effects on which SCEV blocks, but for which it is still clear that the
+; return value remains invariant throughout the whole loop.
+  %ptr = call float* @getNextBasePtr(float* %A)
+  %conv = sitofp i64 %indvar.i to float
+  %arrayidx5 = getelementptr float* %ptr, i64 %indvar.i
+  store float %conv, float* %arrayidx5, align 4
+  store float 1.0, float* %B
+  br label %for.i.inc
+
+for.i.inc:
+  %indvar.i.next = add i64 %indvar.i, 1
+  %exitcond.i = icmp ne i64 %indvar.i.next, %n
+  br i1 %exitcond.i, label %for.i, label %exit
+
+exit:
+  ret void
+}
+
+; CHECK-NOT: Valid Region for Scop