Reuse incremental JIT compilation for --image-codegen (#50649)

We don't need to merge all of the workqueue modules when performing
compilation with `--image-codegen` set, we just need the global variable
initializers to be defined before they're used in one of the modules.
Therefore we can do this by compiling all of the global variable
initializers upfront, so that later references will link them properly.

src/jitlayers.cpp

@@ -141,11 +141,12 @@ void jl_link_global(GlobalVariable *GV, void *addr) JL_NOTSAFEPOINT
     ++LinkedGlobals;
     Constant *P = literal_static_pointer_val(addr, GV->getValueType());
     GV->setInitializer(P);
+    GV->setDSOLocal(true);
     if (jl_options.image_codegen) {
         // If we are forcing imaging mode codegen for debugging,
         // emit external non-const symbol to avoid LLVM optimizing the code
         // similar to non-imaging mode.
-        GV->setLinkage(GlobalValue::ExternalLinkage);
+        assert(GV->hasExternalLinkage());
     }
     else {
         GV->setConstant(true);
@@ -162,6 +163,23 @@ void jl_jit_globals(std::map<void *, GlobalVariable*> &globals) JL_NOTSAFEPOINT
     }
 }
 
+// used for image_codegen, where we keep all the gvs external
+// so we can't jit them directly into each module
+static orc::ThreadSafeModule jl_get_globals_module(orc::ThreadSafeContext &ctx, bool imaging_mode, const DataLayout &DL, const Triple &T, std::map<void *, GlobalVariable*> &globals) JL_NOTSAFEPOINT
+{
+    auto lock = ctx.getLock();
+    auto GTSM = jl_create_ts_module("globals", ctx, imaging_mode, DL, T);
+    auto GM = GTSM.getModuleUnlocked();
+    for (auto &global : globals) {
+        auto GV = global.second;
+        auto GV2 = new GlobalVariable(*GM, GV->getValueType(), GV->isConstant(), GlobalValue::ExternalLinkage, literal_static_pointer_val(global.first, GV->getValueType()), GV->getName(), nullptr, GV->getThreadLocalMode(), GV->getAddressSpace(), false);
+        GV2->copyAttributesFrom(GV);
+        GV2->setDSOLocal(true);
+        GV2->setAlignment(GV->getAlign());
+    }
+    return GTSM;
+}
+
 // this generates llvm code for the lambda info
 // and adds the result to the jitlayers
 // (and the shadow module),
@@ -211,46 +229,53 @@ static jl_callptr_t _jl_compile_codeinst(
 
         if (params._shared_module)
             jl_ExecutionEngine->addModule(orc::ThreadSafeModule(std::move(params._shared_module), params.tsctx));
-        if (!params.imaging) {
-            StringMap<orc::ThreadSafeModule*> NewExports;
+
+        // In imaging mode, we can't inline global variable initializers in order to preserve
+        // the fiction that we don't know what loads from the global will return. Thus, we
+        // need to emit a separate module for the globals before any functions are compiled,
+        // to ensure that the globals are defined when they are compiled.
+        if (params.imaging) {
+            jl_ExecutionEngine->addModule(jl_get_globals_module(params.tsctx, params.imaging, params.DL, params.TargetTriple, params.global_targets));
+        } else {
             StringMap<void*> NewGlobals;
             for (auto &global : params.global_targets) {
                 NewGlobals[global.second->getName()] = global.first;
             }
             for (auto &def : emitted) {
-                orc::ThreadSafeModule &TSM = std::get<0>(def.second);
-                //The underlying context object is still locked because params is not destroyed yet
-                auto M = TSM.getModuleUnlocked();
-                for (auto &F : M->global_objects()) {
-                    if (!F.isDeclaration() && F.getLinkage() == GlobalValue::ExternalLinkage) {
-                        NewExports[F.getName()] = &TSM;
-                    }
-                }
-                // Let's link all globals here also (for now)
+                auto M = std::get<0>(def.second).getModuleUnlocked();
                 for (auto &GV : M->globals()) {
                     auto InitValue = NewGlobals.find(GV.getName());
                     if (InitValue != NewGlobals.end()) {
                         jl_link_global(&GV, InitValue->second);
                     }
                 }
             }
-            DenseMap<orc::ThreadSafeModule*, int> Queued;
-            std::vector<orc::ThreadSafeModule*> Stack;
-            for (auto &def : emitted) {
-                // Add the results to the execution engine now
-                orc::ThreadSafeModule &M = std::get<0>(def.second);
-                jl_add_to_ee(M, NewExports, Queued, Stack);
-                assert(Queued.empty() && Stack.empty() && !M);
-            }
-        } else {
-            jl_jit_globals(params.global_targets);
-            auto main = std::move(emitted[codeinst].first);
-            for (auto &def : emitted) {
-                if (def.first != codeinst) {
-                    jl_merge_module(main, std::move(def.second.first));
+        }
+
+        // Collect the exported functions from the emitted modules,
+        // which form dependencies on which functions need to be
+        // compiled first. Cycles of functions are compiled together.
+        // (essentially we compile a DAG of SCCs in reverse topological order,
+        // if we treat declarations of external functions as edges from declaration
+        // to definition)
+        StringMap<orc::ThreadSafeModule*> NewExports;
+        for (auto &def : emitted) {
+            orc::ThreadSafeModule &TSM = std::get<0>(def.second);
+            //The underlying context object is still locked because params is not destroyed yet
+            auto M = TSM.getModuleUnlocked();
+            for (auto &F : M->global_objects()) {
+                if (!F.isDeclaration() && F.getLinkage() == GlobalValue::ExternalLinkage) {
+                    NewExports[F.getName()] = &TSM;
                 }
             }
-            jl_ExecutionEngine->addModule(std::move(main));
+        }
+        DenseMap<orc::ThreadSafeModule*, int> Queued;
+        std::vector<orc::ThreadSafeModule*> Stack;
+        for (auto &def : emitted) {
+            // Add the results to the execution engine now
+            orc::ThreadSafeModule &M = std::get<0>(def.second);
+            jl_add_to_ee(M, NewExports, Queued, Stack);
+            assert(Queued.empty() && Stack.empty() && !M);
         }
         ++CompiledCodeinsts;
         MaxWorkqueueSize.updateMax(emitted.size());

test/llvmpasses/image-codegen.jl

@@ -13,7 +13,8 @@
 # CHECK-NOT: internal global
 # CHECK-NOT: private global
 # CHECK: jl_global
-# CHECK-SAME: = global
+# COM: we emit both declarations and definitions, so we may see either style in the IR
+# CHECK-SAME: = {{(external )?}}global
 # CHECK: julia_f_
 # CHECK-NOT: internal global
 # CHECK-NOT: private global