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

src/coreclr/tools/superpmi/superpmi-shared/compileresult.cpp

@@ -821,15 +821,15 @@ void CompileResult::applyRelocs(RelocContext* rc, unsigned char* block1, ULONG b
 
                 case CorInfoReloc::ARM32_THUMB_BRANCH24:
                 {
-                    INT32 delta = (INT32)(tmp.target - fixupLocation);
                     if ((section_begin <= address) && (address < section_end)) // A reloc for our section?
                     {
-                        if (!FitsInThumb2BlRel24(delta))
-                        {
-                            DWORDLONG target = (DWORDLONG)originalAddr + (DWORDLONG)blocksize1;
-                            delta            = (INT32)(target - fixupLocation);
-                        }
-                        PutThumb2BlRel24((UINT16*)address, delta);
+                        // Like the arm64 ARM64_BRANCH26 and x64 RELATIVE32 handling, hardcode the
+                        // bottom bits of the target into the instruction so the encoding does not
+                        // depend on where SuperPMI allocated the code buffer. Otherwise a BL whose
+                        // target is out of the +-16MB range for one of the two compared blocks would
+                        // get a buffer-dependent placeholder, producing spurious asm diffs.
+                        DWORDLONG target = tmp.target + (int32_t)tmp.addlDelta;
+                        PutThumb2BlRel24((UINT16*)address, (INT32)(target & 0x00FFFFFE));
                     }
                     wasRelocHandled = true;
                 }

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/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;
+                    }
                 }
             }