SuperPMI: fix spurious arm asmdiffs from out-of-range BL relocs

src/coreclr/tools/superpmi/superpmi/jitinstance.h

@@ -52,6 +52,10 @@ class JitInstance
     ULONGLONG        times[2];
     ICorJitCompiler* pJitInstance;
 
+    // The loaded JIT module handle. Used to detect when the baseline and diff
+    // JITs resolve to the same loaded module (which shares global state).
+    HMODULE getModule() const { return hLib; }
+
     // Allocate and initialize the jit provided
     static JitInstance* InitJit(char*          nameOfJit,
                                 bool           breakOnAssert,

src/coreclr/tools/superpmi/superpmi/neardiffer.cpp

@@ -562,6 +562,28 @@ bool NearDiffer::compareOffsets(
         return compareOffsetsWasm(payload, blockOffset, instrLen, offset1, offset2);
     }
 
+    // On arm32 a direct BL is fixed up with an ARM32_THUMB_BRANCH24 reloc. When the
+    // two code blocks are allocated more than +-16MB apart, applyRelocs substitutes a
+    // placeholder target for the out-of-range side, so the immediates differ even
+    // though both BLs target the same recorded address. Treat the sites as equal when
+    // both recorded the same reloc kind and target.
+    if (GetSpmiTargetArchitecture() == SPMI_TARGET_ARCHITECTURE_ARM)
+    {
+        const DiffData* data = (const DiffData*)payload;
+
+        const Agnostic_RecordRelocation* reloc1 =
+            data->cr1->findRelocationInRange(data->originalBlock1, blockOffset, instrLen);
+        const Agnostic_RecordRelocation* reloc2 =
+            data->cr2->findRelocationInRange(data->originalBlock2, blockOffset, instrLen);
+
+        if ((reloc1 != nullptr) && (reloc2 != nullptr) && (reloc1->fRelocType == reloc2->fRelocType) &&
+            ((uint64_t)reloc1->target + (int32_t)reloc1->addlDelta ==
+             (uint64_t)reloc2->target + (int32_t)reloc2->addlDelta))
+        {
+            return true;
+        }
+    }
+
     const DiffData* data         = (const DiffData*)payload;
     size_t          ip1          = data->originalBlock1 + blockOffset;
     size_t          ip2          = data->originalBlock2 + blockOffset;

src/coreclr/tools/superpmi/superpmi/superpmi.cpp

@@ -447,6 +447,18 @@ int __cdecl main(int argc, char* argv[])
                         // InitJit already printed a failure message
                         return (int)SpmiResult::JitFailedToInit;
                     }
+
+                    if (jit2->getModule() == jit->getModule())
+                    {
+                        // The baseline and diff JITs resolved to the same loaded module. Because the JIT keeps
+                        // global state (e.g. g_jitHost, JitConfig), sharing a single module between the two
+                        // JitInstances corrupts that state and produces spurious diffs and intermittent crashes.
+                        // Require the two JITs to be distinct files (copy one to a different path if needed).
+                        LogError("The baseline JIT ('%s') and diff JIT ('%s') resolve to the same loaded module. "
+                                 "They must be distinct files; copy one JIT to a different path.",
+                                 o.nameOfJit, o.nameOfJit2);
+                        return (int)SpmiResult::JitFailedToInit;
+                    }
                 }
             }