[SCEV] Also use info from assumes in applyLoopGuards.

[fhahn]

Summary

Similar to collecting information from branches guarding a loop, we can
also collect information from assumes dominating the loop header.

Fixes PR47247.

[jdoerfert]

commit message?

llvm/lib/Analysis/ScalarEvolution.cpp

12662

Nit: perform the dyn_cast first, dominance check is probably more costly.

[fhahn]

In D87854#2280261, @jdoerfert wrote:

commit message?

It was not quite ready yet, but now everything should be in place and this patch only contains the important change. I'll update the title/description shortly after updating the diff.

[jdoerfert]

LGTM

llvm/lib/Analysis/ScalarEvolution.cpp

for (std::pair<const BasicBlock *, const BasicBlock *> Pair(
 
     auto Predicate = Cmp->getPredicate();
     if (LoopEntryPredicate->getSuccessor(1) == Pair.second)
       Predicate = CmpInst::getInversePredicate(Predicate);
     CollectCondition(Predicate, getSCEV(Cmp->getOperand(0)),
                      getSCEV(Cmp->getOperand(1)), RewriteMap);
   }
 
+  // Also collect information from assumptions dominating the loop.
+  for (auto &AssumeVH : AC.assumptions()) {
+    if (!AssumeVH)
+      continue;
+    auto *AssumeI = cast<CallInst>(AssumeVH);
+    auto *Cmp = dyn_cast<ICmpInst>(AssumeI->getOperand(0));
+    if (!Cmp || !DT.dominates(AssumeI, L->getHeader()))
+      continue;
+    CollectCondition(Cmp->getPredicate(), getSCEV(Cmp->getOperand(0)),
+                     getSCEV(Cmp->getOperand(1)), RewriteMap);
+  }
+
   if (RewriteMap.empty())
     return Expr;
   return SCEVParameterRewriter::rewrite(Expr, *this, RewriteMap);
 }

llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll

 }
 
 ; Test case for PR47247. Both the guard condition and the assume limit the
 ; max backedge-taken count.
 
 define void @test_guard_and_assume(i32* nocapture readonly %data, i64 %count) {
 ; CHECK-LABEL: @test_guard_and_assume
 ; CHECK:       Loop %loop: backedge-taken count is (-1 + %count)
-; CHECK-NEXT:  Loop %loop: max backedge-taken count is -2
+; CHECK-NEXT:  Loop %loop: max backedge-taken count is 3
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (-1 + %count)
 ;
 entry:
   %cmp = icmp ult i64 %count, 5
   tail call void @llvm.assume(i1 %cmp)
   %cmp18.not = icmp eq i64 %count, 0
   br i1 %cmp18.not, label %exit, label %loop