From de41c51448d6d193145bd24e3f37f19ce6363558 Mon Sep 17 00:00:00 2001
From: Nathan Daly <nathan.daly@relational.ai>
Date: Fri, 31 Jan 2025 15:22:54 -0700
Subject: [PATCH] Extend Base.bitcast(ty, v) to all packed isbits types.

Previously, `Base.bitcast(ty, v)` only supported primitive types.
This commit extends it to support any packed isbits type.
---
 src/datatype.c           |  5 +++++
 src/julia.h              |  5 +++++
 src/julia_internal.h     |  1 +
 src/runtime_intrinsics.c | 12 ++++++++----
 4 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/src/datatype.c b/src/datatype.c
index fd25cca503676..5e906be8ce955 100644
--- a/src/datatype.c
+++ b/src/datatype.c
@@ -371,6 +371,11 @@ int jl_datatype_isinlinealloc(jl_datatype_t *ty, int pointerfree)
     return 0;
 }
 
+JL_DLLEXPORT int jl_datatype_has_padding(jl_datatype_t *dt)
+{
+    return jl_is_datatype(dt) && dt->layout && dt->layout->flags.haspadding;
+}
+
 static unsigned union_isinlinable(jl_value_t *ty, int pointerfree, size_t *nbytes, size_t *align, int asfield)
 {
     if (jl_is_uniontype(ty)) {
diff --git a/src/julia.h b/src/julia.h
index b5416568b7ae9..6553795593b52 100644
--- a/src/julia.h
+++ b/src/julia.h
@@ -1611,6 +1611,11 @@ STATIC_INLINE int jl_is_primitivetype(void *v) JL_NOTSAFEPOINT
     return (jl_is_datatype(v) && ((jl_datatype_t*)(v))->isprimitivetype);
 }
 
+STATIC_INLINE int jl_is_bitstype(void *v) JL_NOTSAFEPOINT
+{
+    return (jl_is_datatype(v) && ((jl_datatype_t*)(v))->isbitstype);
+}
+
 STATIC_INLINE int jl_is_structtype(void *v) JL_NOTSAFEPOINT
 {
     return (jl_is_datatype(v) &&
diff --git a/src/julia_internal.h b/src/julia_internal.h
index 3e4967c9d4dca..89fe971debbe0 100644
--- a/src/julia_internal.h
+++ b/src/julia_internal.h
@@ -836,6 +836,7 @@ void jl_init_main_module(void);
 JL_DLLEXPORT int jl_is_submodule(jl_module_t *child, jl_module_t *parent) JL_NOTSAFEPOINT;
 jl_array_t *jl_get_loaded_modules(void);
 JL_DLLEXPORT int jl_datatype_isinlinealloc(jl_datatype_t *ty, int pointerfree);
+JL_DLLEXPORT int jl_datatype_has_padding(jl_datatype_t *dt);
 int jl_type_equality_is_identity(jl_value_t *t1, jl_value_t *t2) JL_NOTSAFEPOINT;
 
 JL_DLLEXPORT void jl_eval_const_decl(jl_module_t *m, jl_value_t *arg, jl_value_t *val);
diff --git a/src/runtime_intrinsics.c b/src/runtime_intrinsics.c
index 1790b9bd8d106..cba8efc261e0d 100644
--- a/src/runtime_intrinsics.c
+++ b/src/runtime_intrinsics.c
@@ -392,10 +392,14 @@ JL_DLLEXPORT BFLOAT16_TYPE julia__truncdfbf2(double param) JL_NOTSAFEPOINT
 JL_DLLEXPORT jl_value_t *jl_bitcast(jl_value_t *ty, jl_value_t *v)
 {
     JL_TYPECHK(bitcast, datatype, ty);
-    if (!jl_is_concrete_type(ty) || !jl_is_primitivetype(ty))
-        jl_error("bitcast: target type not a leaf primitive type");
-    if (!jl_is_primitivetype(jl_typeof(v)))
-        jl_error("bitcast: value not a primitive type");
+    if (!jl_is_concrete_type(ty) || !jl_is_bitstype(ty))
+        jl_error("bitcast: target type not a leaf bits type");
+    if (!jl_is_bitstype(jl_typeof(v)))
+        jl_error("bitcast: value not a bits type");
+    if (jl_datatype_has_padding((jl_datatype_t*)(ty)))
+        jl_error("bitcast: target type has padding. Can only bitcast to packed types.");
+    if (jl_datatype_has_padding((jl_datatype_t*)(jl_typeof(v))))
+        jl_error("bitcast: value struct has padding. Can only bitcast from packed types.");
     if (jl_datatype_size(jl_typeof(v)) != jl_datatype_size(ty))
         jl_error("bitcast: argument size does not match size of target type");
     if (ty == jl_typeof(v))