From 4b3cf118b11ed5ecc623c4ae12213a5aadb10c39 Mon Sep 17 00:00:00 2001
From: Julian P Samaroo <jpsamaroo@jpsamaroo.me>
Date: Wed, 9 Jun 2021 14:04:55 -0500
Subject: [PATCH] Temporarily externalize Julia globals

---
 src/driver.jl | 37 +++++++++++++++++++++++++++++++++++++
 src/mcgen.jl  |  4 ++++
 2 files changed, 41 insertions(+)

diff --git a/src/driver.jl b/src/driver.jl
index 9f36d720..2dd00514 100644
--- a/src/driver.jl
+++ b/src/driver.jl
@@ -219,8 +219,12 @@ const __llvm_initialized = Ref(false)
                 internalize!(pm, exports)
 
                 # eliminate all unused internal functions
+                add!(pm, ModulePass("ExternalizeJuliaGlobals",
+                                    externalize_julia_globals!))
                 global_optimizer!(pm)
                 global_dce!(pm)
+                add!(pm, ModulePass("InternalizeJuliaGlobals",
+                                    internalize_julia_globals!))
                 strip_dead_prototypes!(pm)
 
                 # merge constants (such as exception messages) from the runtime
@@ -309,6 +313,39 @@ const __llvm_initialized = Ref(false)
     return ir, (; entry, compiled)
 end
 
+# Protect null globals from being killed and inlined
+function externalize_julia_globals!(mod::LLVM.Module)
+    changed = false
+    for gbl in LLVM.globals(mod)
+        if LLVM.linkage(gbl) == LLVM.API.LLVMInternalLinkage &&
+           typeof(LLVM.initializer(gbl)) <: LLVM.PointerNull &&
+           (startswith(LLVM.name(gbl), "jl_global") ||
+            startswith(LLVM.name(gbl), "jl_sym"))
+            LLVM.linkage!(gbl, LLVM.API.LLVMExternalLinkage)
+            LLVM.initializer!(gbl, nothing)
+            LLVM.extinit!(gbl, true)
+            changed = true
+        end
+    end
+    changed
+end
+# And reset the back later
+function internalize_julia_globals!(mod::LLVM.Module)
+    changed = false
+    for gbl in LLVM.globals(mod)
+        if LLVM.linkage(gbl) == LLVM.API.LLVMExternalLinkage &&
+           LLVM.initializer(gbl) === nothing &&
+           (startswith(LLVM.name(gbl), "jl_global") ||
+            startswith(LLVM.name(gbl), "jl_sym"))
+            LLVM.extinit!(gbl, false)
+            LLVM.initializer!(gbl, null(eltype(llvmtype(gbl))))
+            LLVM.linkage!(gbl, LLVM.API.LLVMInternalLinkage)
+            changed = true
+        end
+    end
+    changed
+end
+
 @locked function emit_asm(@nospecialize(job::CompilerJob), ir::LLVM.Module;
                           strip::Bool=false, validate::Bool=true, format::LLVM.API.LLVMCodeGenFileType)
     finish_module!(job, ir)
diff --git a/src/mcgen.jl b/src/mcgen.jl
index ba28e787..404abb39 100644
--- a/src/mcgen.jl
+++ b/src/mcgen.jl
@@ -7,11 +7,15 @@ function prepare_execution!(@nospecialize(job::CompilerJob), mod::LLVM.Module)
         global current_job
         current_job = job
 
+        add!(pm, ModulePass("ExternalizeJuliaGlobals",
+                            externalize_julia_globals!))
         global_optimizer!(pm)
 
         add!(pm, ModulePass("ResolveCPUReferences", resolve_cpu_references!))
 
         global_dce!(pm)
+        add!(pm, ModulePass("InternalizeJuliaGlobals",
+                            internalize_julia_globals!))
         strip_dead_prototypes!(pm)
 
         run!(pm, mod)