[IR][TRE] Support associative intrinsics (#74226)

There is support for intrinsics in Instruction::isCommunative, but there
is no equivalent implementation for isAssociative. This patch builds
support for associative intrinsics with TRE as an application. TRE can
now have associative intrinsics as an accumulator. For example:
```
struct Node {
  Node *next;
  unsigned val;
}

unsigned maxval(struct Node *n) {
  if (!n) return 0;
  return std::max(n->val, maxval(n->next));
}
```
Can be transformed into:
```
unsigned maxval(struct Node *n) {
  struct Node *head = n;
  unsigned max = 0; // Identity of unsigned std::max
  while (true) {
    if (!head) return max;
    max = std::max(max, head->val);
    head = head->next;
  }
  return max;
}
```
This example results in about 5x speedup in local runs.

We conservatively only consider min/max and as associative for this
patch to limit testing scope. There are probably other intrinsics that
could be considered associative. There are a few consumers of
isAssociative() that could be impacted. Testing has only required to
Reassociate pass be updated.

1 files changed, 26 insertions, 0 deletions

diff --git a/llvm/lib/IR/Constants.cpp b/llvm/lib/IR/Constants.cpp
index bc55d5b..a38b912 100644
--- a/llvm/lib/IR/Constants.cpp
+++ b/llvm/lib/IR/Constants.cpp
@@ -2556,6 +2556,32 @@ Constant *ConstantExpr::getBinOpIdentity(unsigned Opcode, Type *Ty,
   }
 }
 
+Constant *ConstantExpr::getIntrinsicIdentity(Intrinsic::ID ID, Type *Ty) {
+  switch (ID) {
+  case Intrinsic::umax:
+    return Constant::getNullValue(Ty);
+  case Intrinsic::umin:
+    return Constant::getAllOnesValue(Ty);
+  case Intrinsic::smax:
+    return Constant::getIntegerValue(
+        Ty, APInt::getSignedMinValue(Ty->getIntegerBitWidth()));
+  case Intrinsic::smin:
+    return Constant::getIntegerValue(
+        Ty, APInt::getSignedMaxValue(Ty->getIntegerBitWidth()));
+  default:
+    return nullptr;
+  }
+}
+
+Constant *ConstantExpr::getIdentity(Instruction *I, Type *Ty,
+                                    bool AllowRHSConstant, bool NSZ) {
+  if (I->isBinaryOp())
+    return getBinOpIdentity(I->getOpcode(), Ty, AllowRHSConstant, NSZ);
+  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
+    return getIntrinsicIdentity(II->getIntrinsicID(), Ty);
+  return nullptr;
+}
+
 Constant *ConstantExpr::getBinOpAbsorber(unsigned Opcode, Type *Ty) {
   switch (Opcode) {
   default: