[AMDGPU] AMDGPUAAResult::pointsToConstantMemory should not use the default MaxLookup (i.e., 6) to limit getUnderlyingObject

[jizhuoran]

Summary

The default value of MaxLookup is 6, which limits the number of instructions to be stripped off when getting the underlying object. 'Loop Unroll' and 'Loop Strength Reduction' passes trend to replace the memory index 'base + i * offset' to 'base, base += offset, base += offset'. It increases the depth of the underlying object so that pointsToConstantMemory may fail to identify a pointer's underlying object is a NoAlias and ReadOnly Argument. It leads to false memory load scheduling dependency and prevents the instruction scheduler to pipeline the memory load operations.

The default MaxLookup is too small for this case. In general, even the first memory load has a GEP and a bitcast. The false memory dependency begins from the fifth memory load for a global memory (argument). It causes less efficient assembly codes.a

Specifying a larger MaxLookup value or even an unlimited MaxLookup (i.e., 0) solves this problem.

[rampitec]

Unlimited is really too aggressive. It can slow down compilation dramatically in some cases.
Also it would be nice to see a relevant test.

[alex-t]

I would agree with Stas here.
In case you can identify the patterns that require the lookup deeper then 6 levels, you probably can formulate the exact threshold.
And adding tests for such a pattern would make it clear.

[arsenm]

Unbounding this completely is too aggressive, and needs a test case

llvm/lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp

 //===- AMDGPUAliasAnalysis ------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 /// \file

 
 bool AMDGPUAAResult::pointsToConstantMemory(const MemoryLocation &Loc,
                                             AAQueryInfo &AAQI, bool OrLocal) {
   unsigned AS = Loc.Ptr->getType()->getPointerAddressSpace();
   if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
       AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT)
     return true;
 
-  const Value *Base = getUnderlyingObject(Loc.Ptr);
+  const Value *Base = getUnderlyingObject(Loc.Ptr, 0);
   AS = Base->getType()->getPointerAddressSpace();
   if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
       AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT)
     return true;
 
   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
     if (GV->isConstant())
       return true;