Fix Windows LTO: preserve required procedures with llvm.used

On Windows with LTO, required procedures with external linkage need to
be added to @llvm.used to survive linker-level dead code elimination.

LLVM may generate implicit calls to runtime builtins (e.g., __extendhfsf2
for f16 conversions) during instruction lowering, after the IR is
finalized. Without @llvm.used, the linker discards these procedures
before the implicit calls are generated.

This adds required procedures to @llvm.used at creation time. The fix
is Windows-specific; other platforms handle this correctly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/llvm_backend_proc.cpp b/src/llvm_backend_proc.cpp
index 837d7ce48..cb8ffcf91 100644
--- a/src/llvm_backend_proc.cpp
+++ b/src/llvm_backend_proc.cpp
@@ -289,6 +289,19 @@ gb_internal lbProcedure *lb_create_procedure(lbModule *m, Entity *entity, bool i
 
 	lb_set_linkage_from_entity_flags(p->module, p->value, entity->flags);
 
+	// With LTO on Windows, required procedures with external linkage need to be added to
+	// llvm.used to survive linker-level dead code elimination. This is necessary because
+	// LLVM may generate implicit calls to runtime builtins (e.g., __extendhfsf2 for f16
+	// conversions) during instruction lowering, after the IR is finalized.
+	if (build_context.lto_kind != LTO_None && build_context.metrics.os == TargetOs_windows) {
+		if (entity->flags & EntityFlag_Require) {
+			LLVMLinkage linkage = LLVMGetLinkage(p->value);
+			if (linkage != LLVMInternalLinkage) {
+				lb_append_to_used(m, p->value);
+			}
+		}
+	}
+
 	if (m->debug_builder) { // Debug Information
 		Type *bt = base_type(p->type);