Make 'UnrollMaxUpperBound' to be overridable by target.

[boxu-zhang]

The UnrollMaxUpperBound should be target dependent, since different chips provide different register set which brings different ability of storing more temporary values of a program. So I add a MaxUpperBound value in UnrollingPreference which can be override by targets. All uses of UnrollMaxUpperBound are replaced with UP.MaxUpperBound.

The default value is still 8 and the command line argument '--unroll-max-upperbound' takes final effect if provided.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

@llvm/pr-subscribers-llvm-transforms

Author: boxu.zhang (boxu-zhang)

Changes

The UnrollMaxUpperBound should be target dependent, since different chips provide different register set which brings different ability of storing more temporary values of a program. So I add a MaxUpperBound value in UnrollingPreference which can be override by targets. All uses of UnrollMaxUpperBound are replaced with UP.MaxUpperBound.

The default value is still 8 and the command line argument '--unroll-max-upperbound' takes final effect if provided.

---

Full diff: https://github.com/llvm/llvm-project/pull/76029.diff

2 Files Affected:

• (modified) llvm/include/llvm/Analysis/TargetTransformInfo.h (+7)
• (modified) llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp (+6-3)

diff --git a/llvm/include/llvm/Analysis/TargetTransformInfo.h b/llvm/include/llvm/Analysis/TargetTransformInfo.h
index f5114fa40c70ad..3d7191b902b0ea 100644
--- a/llvm/include/llvm/Analysis/TargetTransformInfo.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfo.h
@@ -560,6 +560,13 @@ class TargetTransformInfo {
     // (set to UINT_MAX to disable). This does not apply in cases where the
     // loop is being fully unrolled.
     unsigned MaxCount;
+    /// Set the maximum upper bound of trip count. Considering a loop with 'pragma 
+    /// unroll' specified but without 'full' or 'n' field, it depends on the 
+    /// UnrollMaxUpperBound option if the loop trip is unknown. However, the 
+    /// default value is 8 for this option which might not suitable for certain
+    /// target. Allowing the MaxUpperBound to be overrided by a target gives more
+    /// flexiblity on such cases. By default, MaxUpperBound uses UnrollMaxUpperBound
+    unsigned MaxUpperBound;
     /// Set the maximum unrolling factor for full unrolling. Like MaxCount, but
     /// applies even if full unrolling is selected. This allows a target to fall
     /// back to Partial unrolling if full unrolling is above FullUnrollMaxCount.
diff --git a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
index f14541a1a037e6..ddc532ea24e275 100644
--- a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -200,6 +200,7 @@ TargetTransformInfo::UnrollingPreferences llvm::gatherUnrollingPreferences(
   UP.Count = 0;
   UP.DefaultUnrollRuntimeCount = 8;
   UP.MaxCount = std::numeric_limits<unsigned>::max();
+  UP.MaxUpperBound = UnrollMaxUpperBound; 
   UP.FullUnrollMaxCount = std::numeric_limits<unsigned>::max();
   UP.BEInsns = 2;
   UP.Partial = false;
@@ -237,6 +238,8 @@ TargetTransformInfo::UnrollingPreferences llvm::gatherUnrollingPreferences(
     UP.MaxPercentThresholdBoost = UnrollMaxPercentThresholdBoost;
   if (UnrollMaxCount.getNumOccurrences() > 0)
     UP.MaxCount = UnrollMaxCount;
+  if (UnrollMaxUpperBound.getNumOccurrences() > 0)
+    UP.MaxUpperBound = UnrollMaxUpperBound;
   if (UnrollFullMaxCount.getNumOccurrences() > 0)
     UP.FullUnrollMaxCount = UnrollFullMaxCount;
   if (UnrollAllowPartial.getNumOccurrences() > 0)
@@ -777,7 +780,7 @@ shouldPragmaUnroll(Loop *L, const PragmaInfo &PInfo,
     return TripCount;
 
   if (PInfo.PragmaEnableUnroll && !TripCount && MaxTripCount &&
-      MaxTripCount <= UnrollMaxUpperBound)
+      MaxTripCount <= UP.MaxUpperBound)
     return MaxTripCount;
 
   // if didn't return until here, should continue to other priorties
@@ -952,7 +955,7 @@ bool llvm::computeUnrollCount(
   // cost of exact full unrolling.  As such, if we have an exact count and
   // found it unprofitable, we'll never chose to bounded unroll.
   if (!TripCount && MaxTripCount && (UP.UpperBound || MaxOrZero) &&
-      MaxTripCount <= UnrollMaxUpperBound) {
+      MaxTripCount <= UP.MaxUpperBound) {
     UP.Count = MaxTripCount;
     if (auto UnrollFactor = shouldFullUnroll(L, TTI, DT, SE, EphValues,
                                              MaxTripCount, UCE, UP)) {
@@ -1026,7 +1029,7 @@ bool llvm::computeUnrollCount(
   }
 
   // Don't unroll a small upper bound loop unless user or TTI asked to do so.
-  if (MaxTripCount && !UP.Force && MaxTripCount < UnrollMaxUpperBound) {
+  if (MaxTripCount && !UP.Force && MaxTripCount < UP.MaxUpperBound) {
     UP.Count = 0;
     return false;
   }

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[nikic]

Making this a TTI hook sounds reasonable to me.

[nikic]

Suggested change

	/// Set the maximum upper bound of trip count. Considering a loop with 'pragma
	/// unroll' specified but without 'full' or 'n' field, it depends on the
	/// UnrollMaxUpperBound option if the loop trip is unknown. However, the
	/// default value is 8 for this option which might not suitable for certain
	/// target. Allowing the MaxUpperBound to be overrided by a target gives more
	/// flexiblity on such cases. By default, MaxUpperBound uses UnrollMaxUpperBound
	/// Set the maximum number of iterations to fully unroll if only an upper bound
	/// on the trip count is known.

I don't think the rest of the discussion really belongs in this comment. Note that this option also affects the non-pragma case, so mentioning that is misleading.

[boxu-zhang]

Indeed.

[boxu-zhang]

Comments update, can you please review it again and merge if it's OK?

[boxu-zhang]

It's OK now. Can you please merge it?

[nikic]

Can you please fix the clang-format failure? It's due to trailing whitespace.

[boxu-zhang]

Can you please fix the clang-format failure? It's due to trailing whitespace.

Updated.