Use method symbol address to get the entry point in Tril and JitBuilder

Some platforms, such as AIX and z/OS, use metadata structures to
represent "pointer to a function" retrieved via the C `&` operator. On
AIX and z/OS these are called Function Descriptors. We need to
disambiguate between the "pointer to a function" and "address of entry
point of a function".

Currently `getCodeStart` API returns the latter, however this is not
what we want when we query for the result of a compilation in Tril and
JitBuilder. We want the former, i.e. the value of the "pointer to a
function" of the JIT compiled method.

We use the method symbol `getAddress` API here to query for such
values, and ensuring this value is set properly after binary encoding.
It is currently set to the result of `getCodeStart` by default, so no
change in value. If platforms have Function Descriptors it is up to
those linkages to update this value once the location of the Function
Descriptor is known.

Signed-off-by: Filip Jeremic <fjeremic@ca.ibm.com>

Signed-off-by: Filip Jeremic <fjeremic@ca.ibm.com>

compiler/codegen/OMRCodeGenPhase.cpp

@@ -196,11 +196,6 @@ OMR::CodeGenPhase::performProcessRelocationsPhase(TR::CodeGenerator * cg, TR::Co
         diagnostic("%08d   %s\n", cg->getCodeLength(), comp->signature());
         }
 
-     if (comp->getCurrentMethod() == NULL)
-        {
-        comp->getMethodSymbol()->setMethodAddress(cg->getBinaryBufferStart());
-        }
-
      TR_ASSERT(cg->getCodeLength() <= cg->getEstimatedCodeLength(),
                "Method length estimate must be conservatively large\n"
                "    codeLength = %d, estimatedCodeLength = %d \n",
@@ -304,6 +299,9 @@ OMR::CodeGenPhase::performBinaryEncodingPhase(TR::CodeGenerator * cg, TR::CodeGe
 
    cg->doBinaryEncoding();
 
+   // Instructions have been emitted, and now we know what the entry point is, so update the compilation method symbol
+   comp->getMethodSymbol()->setMethodAddress(cg->getCodeStart());
+
    if (debug("verifyFinalNodeReferenceCounts"))
       {
       if (cg->getDebug())

compiler/control/CompileMethod.cpp

@@ -379,7 +379,7 @@ compileMethodFromDetails(
          // not ready yet...
          //OMR::MethodMetaDataPOD *metaData = fe.createMethodMetaData(&compiler);
 
-         startPC = compiler.cg()->getCodeStart();
+         startPC = (uint8_t*)compiler.getMethodSymbol()->getMethodAddress();
          uint64_t translationTime = TR::Compiler->vm.getUSecClock() - translationStartTime;
 
          if (TR::Options::isAnyVerboseOptionSet(TR_VerboseCompileEnd, TR_VerbosePerformance))

compiler/z/codegen/SystemLinkagezOS.cpp

@@ -1163,6 +1163,9 @@ TR::S390zOSSystemLinkage::XPLINKFunctionDescriptorSnippet::emitSnippetBody()
    // TODO: We should not have to do this here. This should be done by the caller.
    getSnippetLabel()->setCodeLocation(cursor);
 
+   // Update the method symbol address to point to the function descriptor
+   cg()->comp()->getMethodSymbol()->setMethodAddress(cursor);
+
    if (cg()->comp()->target().is32Bit())
       {
       // Address of function's environment

fvtest/tril/tril/simple_compiler.cpp

@@ -83,19 +83,7 @@ int32_t Tril::SimpleCompiler::compileWithVerifier(TR::IlVerifier* verifier) {
     // if compilation was successful, set the entry point for the compiled body
     if (rc == 0)
        {
-#if defined(J9ZOS390)
-       struct FunctionDescriptor
-       {
-          uint64_t environment;
-          void* func;
-       };
-
-       FunctionDescriptor* fd = new FunctionDescriptor();
-       fd->environment = 0;
-       fd->func = entry_point;
-
-       entry_point = (uint8_t*) fd;
-#elif defined(AIXPPC)
+#if defined(AIXPPC)
        struct FunctionDescriptor
           {
           void* func;

jitbuilder/control/Jit.cpp

@@ -189,19 +189,7 @@ internal_compileMethodBuilder(TR::MethodBuilder *m, void **entry)
    {
    auto rc = m->Compile(entry);
 
-#if defined(J9ZOS390)
-   struct FunctionDescriptor
-   {
-      uint64_t environment;
-      void* func;
-   };
-
-   FunctionDescriptor* fd = new FunctionDescriptor();
-   fd->environment = 0;
-   fd->func = *entry;
-
-   *entry = (void*) fd;
-#elif defined(AIXPPC)
+#if defined(AIXPPC)
    struct FunctionDescriptor
       {
       void* func;