[llvm] Make 'UnrollMaxUpperBound' to be overridable by target. (PR #76029)

[via llvm-commits]

https://github.com/boxu-zhang created https://github.com/llvm/llvm-project/pull/76029

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.

>From 4546bc62664f6c5e1554aa68b5529a7f0f2aefcc Mon Sep 17 00:00:00 2001
From: "boxu.zhang" <boxu.zhang at hotmail.com>
Date: Wed, 20 Dec 2023 17:35:25 +0800
Subject: [PATCH] Make 'UnrollMaxUpperBound' to be overridable by target. The
 default value is still 8 and the command line argument
 '--unroll-max-upperbound' takes final effect if provided.

---
 llvm/include/llvm/Analysis/TargetTransformInfo.h | 7 +++++++
 llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp    | 9 ++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

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;
   }