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Fix Windows LTO: preserve required procedures with llvm.used

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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>
This commit is contained in:
Jesse Meyer
2026-02-03 20:16:44 -05:00
parent a0562dfd6e
commit bd6148dd6b
2 changed files with 33 additions and 9 deletions
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29
src/llvm_backend_opt.cpp
View File

@@ -512,8 +512,9 @@ gb_internal void llvm_delete_function(LLVMValueRef func) {
LLVMDeleteFunction(func);
}
gb_internal void lb_append_to_compiler_used(lbModule *m, LLVMValueRef value) {
LLVMValueRef global = LLVMGetNamedGlobal(m->mod, "llvm.compiler.used");
// Helper to append a value to an llvm metadata array global (llvm.used or llvm.compiler.used)
gb_internal void lb_append_to_llvm_used_list(lbModule *m, LLVMValueRef value, char const *list_name) {
LLVMValueRef global = LLVMGetNamedGlobal(m->mod, list_name);
LLVMValueRef *constants;
int operands = 1;
@@ -543,33 +544,43 @@ gb_internal void lb_append_to_compiler_used(lbModule *m, LLVMValueRef value) {
constants[operands - 1] = LLVMConstBitCast(value, Int8PtrTy);
LLVMValueRef initializer = LLVMConstArray(Int8PtrTy, constants, operands);
global = LLVMAddGlobal(m->mod, ATy, "llvm.compiler.used");
global = LLVMAddGlobal(m->mod, ATy, list_name);
LLVMSetLinkage(global, LLVMAppendingLinkage);
LLVMSetSection(global, "llvm.metadata");
LLVMSetInitializer(global, initializer);
}
gb_internal void lb_append_to_compiler_used(lbModule *m, LLVMValueRef value) {
lb_append_to_llvm_used_list(m, value, "llvm.compiler.used");
}
// llvm.used survives LTO linker optimizations (unlike llvm.compiler.used)
gb_internal void lb_append_to_used(lbModule *m, LLVMValueRef value) {
lb_append_to_llvm_used_list(m, value, "llvm.used");
}
gb_internal void lb_run_remove_unused_function_pass(lbModule *m) {
isize removal_count = 0;
isize pass_count = 0;
isize const max_pass_count = 10;
// Custom remove dead function pass
// Custom remove dead function pass (for internal linkage functions)
for (; pass_count < max_pass_count; pass_count++) {
bool was_dead = false;
bool was_dead = false;
for (LLVMValueRef func = LLVMGetFirstFunction(m->mod);
func != nullptr;
/**/
) {
LLVMValueRef curr_func = func;
func = LLVMGetNextFunction(func);
LLVMUseRef first_use = LLVMGetFirstUse(curr_func);
if (first_use != nullptr) {
continue;
}
String name = {};
name.text = cast(u8 *)LLVMGetValueName2(curr_func, cast(size_t *)&name.len);
if (LLVMIsDeclaration(curr_func)) {
// Ignore for the time being
continue;
@@ -578,7 +589,7 @@ gb_internal void lb_run_remove_unused_function_pass(lbModule *m) {
if (linkage != LLVMInternalLinkage) {
continue;
}
Entity **found = map_get(&m->procedure_values, curr_func);
if (found && *found) {
Entity *e = *found;
@@ -588,7 +599,7 @@ gb_internal void lb_run_remove_unused_function_pass(lbModule *m) {
continue;
}
}
llvm_delete_function(curr_func);
was_dead = true;
removal_count += 1;

13
src/llvm_backend_proc.cpp
View File

@@ -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);