JIT: modify how jit determines when to update a type (#22618)

For single-def locals, the type of a reference seen at the assignment to the
local may be a more specific type than the local's declared type. If so the jit
would prefer to use the assignment type to describe the local's value, as this
will lead to better optimization. For instance in
```
    object x = "a string";  // only assignment to x
```
the jit can optimize better if it models the type of `x` as `string`.

Instead of relying on `mergeClasses` plus some jit-side screening to decide if
the assignment type is a more specific type, implement a new jit interface
method `isMoreSpecificType` that tries to answer this question more directly.

Added a test case with type equivalence that hit asserts.

Closes #22583.

2 files changed, 10 insertions, 0 deletions

diff --git a/src/zap/zapinfo.cpp b/src/zap/zapinfo.cpp
index a56004a6b4..8a372828f0 100644
--- a/src/zap/zapinfo.cpp
+++ b/src/zap/zapinfo.cpp
@@ -3199,6 +3199,13 @@ CORINFO_CLASS_HANDLE ZapInfo::mergeClasses(
     return m_pEEJitInfo->mergeClasses(cls1, cls2);
 }
 
+BOOL ZapInfo::isMoreSpecificType(
+                CORINFO_CLASS_HANDLE cls1,
+                CORINFO_CLASS_HANDLE cls2)
+{
+    return m_pEEJitInfo->isMoreSpecificType(cls1, cls2);
+}
+
 BOOL ZapInfo::shouldEnforceCallvirtRestriction(
         CORINFO_MODULE_HANDLE scopeHnd)
 {
diff --git a/src/zap/zapinfo.h b/src/zap/zapinfo.h
index eadefe8555..962bbdacd4 100644
--- a/src/zap/zapinfo.h
+++ b/src/zap/zapinfo.h
@@ -591,6 +591,9 @@ public:
 
     CORINFO_CLASS_HANDLE mergeClasses(CORINFO_CLASS_HANDLE cls1,
                                 CORINFO_CLASS_HANDLE cls2);
+    BOOL isMoreSpecificType(CORINFO_CLASS_HANDLE cls1,
+                                CORINFO_CLASS_HANDLE cls2);
+
     BOOL shouldEnforceCallvirtRestriction(CORINFO_MODULE_HANDLE scope);
     CORINFO_CLASS_HANDLE getParentType(CORINFO_CLASS_HANDLE  cls);
     CorInfoType getChildType (CORINFO_CLASS_HANDLE       clsHnd,