[AMDGPU] SIInsertHardClause: add configurable clause length limit (#142343)

Add command line and function attribute configuration of hard clause
length limit (within hardware maximum).
This allows performance tuning for shaders which benefit from smaller
clauses.

Author: perlfu

llvm/lib/Target/AMDGPU/SIInsertHardClauses.cpp

@@ -42,6 +42,12 @@ using namespace llvm;
 
 #define DEBUG_TYPE "si-insert-hard-clauses"
 
+static cl::opt<unsigned>
+    HardClauseLengthLimit("amdgpu-hard-clause-length-limit",
+                          cl::desc("Maximum number of memory instructions to "
+                                   "place in the same hard clause"),
+                          cl::Hidden);
+
 namespace {
 
 enum HardClauseType {
@@ -185,11 +191,18 @@ class SIInsertHardClauses {
   }
 
   bool run(MachineFunction &MF) {
-
     ST = &MF.getSubtarget<GCNSubtarget>();
     if (!ST->hasHardClauses())
       return false;
 
+    unsigned MaxClauseLength = MF.getFunction().getFnAttributeAsParsedInteger(
+        "amdgpu-hard-clause-length-limit", 255);
+    if (HardClauseLengthLimit.getNumOccurrences())
+      MaxClauseLength = HardClauseLengthLimit;
+    MaxClauseLength = std::min(MaxClauseLength, ST->maxHardClauseLength());
+    if (MaxClauseLength <= 1)
+      return false;
+
     const SIInstrInfo *SII = ST->getInstrInfo();
     const TargetRegisterInfo *TRI = ST->getRegisterInfo();
 
@@ -212,7 +225,7 @@ class SIInsertHardClauses {
           }
         }
 
-        if (CI.Length == ST->maxHardClauseLength() ||
+        if (CI.Length == MaxClauseLength ||
             (CI.Length && Type != HARDCLAUSE_INTERNAL &&
              Type != HARDCLAUSE_IGNORE &&
              (Type != CI.Type ||