[llvm][IR] Extend BranchWeightMetadata to track provenance of weights (#86609)

This patch implements the changes to LLVM IR discussed in

https://discourse.llvm.org/t/rfc-update-branch-weights-metadata-to-allow-tracking-branch-weight-origins/75032

In this patch, we add an optional field to MD_prof metadata nodes for
branch weights, which can be used to distinguish weights added from
`llvm.expect*` intrinsics from those added via other methods, e.g.
from profiles or inserted by the compiler.

One of the major motivations, is for use with MisExpect diagnostics,
which need to know if branch_weight metadata originates from an
llvm.expect intrinsic. Without that information, we end up checking
branch weights multiple times in the case if ThinLTO + SampleProfiling,
leading to some inaccuracy in how we report MisExpect related
diagnostics to users.

Since we change the format of MD_prof metadata in a fundamental way, we
need to update code handling branch weights in a number of places.

We also update the lang ref for branch weights to reflect the change.

1 files changed, 6 insertions, 3 deletions

diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index e592720..fe2253d 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -104,6 +104,7 @@
 #include "llvm/IR/Module.h"
 #include "llvm/IR/ModuleSlotTracker.h"
 #include "llvm/IR/PassManager.h"
+#include "llvm/IR/ProfDataUtils.h"
 #include "llvm/IR/Statepoint.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Use.h"
@@ -4808,8 +4809,10 @@ void Verifier::visitProfMetadata(Instruction &I, MDNode *MD) {
 
   // Check consistency of !prof branch_weights metadata.
   if (ProfName == "branch_weights") {
+    unsigned int Offset = getBranchWeightOffset(MD);
     if (isa<InvokeInst>(&I)) {
-      Check(MD->getNumOperands() == 2 || MD->getNumOperands() == 3,
+      Check(MD->getNumOperands() == (1 + Offset) ||
+                MD->getNumOperands() == (2 + Offset),
             "Wrong number of InvokeInst branch_weights operands", MD);
     } else {
       unsigned ExpectedNumOperands = 0;
@@ -4829,10 +4832,10 @@ void Verifier::visitProfMetadata(Instruction &I, MDNode *MD) {
         CheckFailed("!prof branch_weights are not allowed for this instruction",
                     MD);
 
-      Check(MD->getNumOperands() == 1 + ExpectedNumOperands,
+      Check(MD->getNumOperands() == Offset + ExpectedNumOperands,
             "Wrong number of operands", MD);
     }
-    for (unsigned i = 1; i < MD->getNumOperands(); ++i) {
+    for (unsigned i = Offset; i < MD->getNumOperands(); ++i) {
       auto &MDO = MD->getOperand(i);
       Check(MDO, "second operand should not be null", MD);
       Check(mdconst::dyn_extract<ConstantInt>(MDO),