[llvm] [LV] Add initial legality checks for ee loops with stores (PR #145663)

[David Sherwood via llvm-commits]

================
@@ -1793,6 +1814,110 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
   return true;
 }
 
+bool LoopVectorizationLegality::canUncountedExitConditionLoadBeMoved(
+    BasicBlock *ExitingBlock) {
+  SmallVector<const SCEVPredicate *, 4> Predicates;
+  LoadInst *CriticalUncountedExitConditionLoad = nullptr;
+
+  // Try to find a load in the critical path for the uncounted exit condition.
+  // This is currently matching about the simplest form we can, expecting
+  // only one in-loop load, the result of which is directly compared against
+  // a loop-invariant value.
+  // FIXME: We're insisting on a single use for now, because otherwise we will
+  // need to make PHI nodes for other users. That can be done once the initial
+  // transform code lands.
+  if (BranchInst *Br = dyn_cast<BranchInst>(ExitingBlock->getTerminator())) {
+    // FIXME: Don't rely on operand ordering for the comparison.
+    ICmpInst *Cmp = dyn_cast<ICmpInst>(Br->getCondition());
+    if (Cmp && Cmp->hasOneUse() &&
+        TheLoop->isLoopInvariant(Cmp->getOperand(1))) {
+      LoadInst *Load = dyn_cast<LoadInst>(Cmp->getOperand(0));
+      if (Load && Load->hasOneUse() && !TheLoop->isLoopInvariant(Load)) {
+        // The following call also checks that the load address is either
+        // invariant or is an affine SCEVAddRecExpr with a constant step.
+        // In either case, we're not relying on another load.
+        // FIXME: Support gathers after first-faulting support lands.
+        if (isDereferenceableAndAlignedInLoop(Load, TheLoop, *PSE.getSE(), *DT,
+                                              AC, &Predicates)) {
+          ICFLoopSafetyInfo SafetyInfo;
+          SafetyInfo.computeLoopSafetyInfo(TheLoop);
+          // We need to know that load will be executed before we can hoist a
+          // copy out to run just before the first iteration.
+          if (SafetyInfo.isGuaranteedToExecute(*Load, DT, TheLoop))
+            CriticalUncountedExitConditionLoad = Load;
+          else
+            reportVectorizationFailure(
+                "Early exit condition load not guaranteed to execute",
+                "Cannot vectorize early exit loop when condition load is not "
+                "guaranteed to execute",
+                "EarlyExitLoadNotGuaranteed", ORE, TheLoop);
+        } else {
+          reportVectorizationFailure(
+              "Loop may fault",
+              "Cannot vectorize potentially faulting early exit loop",
+              "PotentiallyFaultingEarlyExitLoop", ORE, TheLoop);
+          return false;
+        }
+      }
+    }
+  } else {
+    reportVectorizationFailure(
+        "Unsupported control flow in early exit loop with side effects",
+        "Cannot find branch instruction for uncounted exit in early exit loop "
+        "with side effects",
+        "UnsupportedUncountedExitTerminator", ORE, TheLoop);
+    return false;
+  }
+
+  if (!CriticalUncountedExitConditionLoad) {
+    reportVectorizationFailure(
+        "Early exit loop with store but no condition load",
+        "Cannot vectorize early exit loop with store but no condition load",
+        "NoConditionLoadForEarlyExitLoop", ORE, TheLoop);
+    return false;
+  }
+
+  // We're in a bit of an odd spot since we're (potentially) doing the load
+  // out of its normal order in the loop and that may throw off dependency
+  // checking. A forward dependency should be fine, but a backwards dep may not
+  // be even if LAA thinks it is due to performing the load for the vector
+  // iteration i+1 in vector iteration i.
+  // In any case, prohibit vectorization if there are any loop-carried
+  // dependencies on the critical load.
+  // FIXME: Relax this constraint where possible.
+  LAI = &LAIs.getInfo(*TheLoop);
+  const MemoryDepChecker &DepChecker = LAI->getDepChecker();
+  const auto *Deps = DepChecker.getDependences();
+  if (!Deps) {
+    // We may have exceeded the allowed number of dependencies to track, and
+    // given up. Just bail out since we can't be sure.
+    reportVectorizationFailure(
+        "Invalid memory dependencies result",
+        "Unable to determine memory dependencies for an early exit loop with "
+        "side effects.",
+        "CantVectorizeInvalidDependencesForEELoopsWithSideEffects", ORE,
+        TheLoop);
+    return false;
+  }
+
+  if (any_of(*Deps, [&](const MemoryDepChecker::Dependence &Dep) {
+        return (Dep.getDestination(DepChecker) ==
+                    CriticalUncountedExitConditionLoad ||
+                Dep.getSource(DepChecker) ==
+                    CriticalUncountedExitConditionLoad);
+      })) {
+    reportVectorizationFailure(
+        "No dependencies allowed for critical early exit condition load "
----------------
david-arm wrote:

For some of these reports you can use a single string version of `reportVectorizationFailure` if you don't really care about the debug message and report errors being different?

https://github.com/llvm/llvm-project/pull/145663