offset-based cow_size_unsafe defined, dubiously...

include/cow.h

@@ -24,6 +24,14 @@ cow_t* cow_clone(const cow_t* cow);
 int cow_is_fake(const cow_t* cow);
 /// Get the size of this cow area.
 size_t cow_size(const cow_t* cow);
+/// Get the size of this cow area by assuming layout. This should work assuming "cow_t.h"'s build assertions didn't fail and avoids an extra call.
+#ifdef _COW_NO_ASSUME_ABI
+#define cow_size_unsafe(v) cow_size(v)
+#else
+// XXX: This macro is *VERY* ABI sensitive. This shouldn't be used if the ABI has changed since the build of libcow's `cow_t.h` passed its static assertions in *both* the C and C++ implementations.
+// The C++ API uses this by default for its `Cow::size()` function.
+#define cow_size_unsafe(v) *(((size_t*)(v))+1)
+#endif
 
 /// Get the `void*` pointer to the start of the area.
 #define cow_ptr(v) (*((void**)(v)))

include/cow.hpp

@@ -4,6 +4,7 @@
 
 #include <memory>
 
+
 struct Cow {
 	struct Fake;
 	Cow() = delete;
@@ -16,13 +17,29 @@ struct Cow {
 
 	inline void* area() { return cow_ptr(get_raw()); }
 	inline const void* area() const { return cow_ptr_of(const void, get_raw()); }
-	inline size_t size() const { return cow_size(get_raw()); }
+
+	/// Get the size of the mapped area.
+	///
+	/// Note: This calls into `_inner`'s internals. To skip the call on non-LTO builds, use `size_unsafe()`.
+	size_t size() const;
+	/// Get the size by assuming the ABI layout of cow_t to be correct. Potentially faster but ABI sensitive.
+	/// This shouldn't be a problem if all build static assertions passed.
+	///
+	/// Note: This behaviour can be diabled by building with `-DCOW_NO_ASSUME_ABI`. In this case, this function calls out to the C API to determine the size.
+	/// 	  There is also likely no benefit using this over `size()` in LTO enabled builds.
+	inline size_t size_unsafe() const { return cow_size_unsafe(get_raw()); }
 	
 	inline unsigned char* as_bytes() { return (unsigned char*)area(); }
 	inline const unsigned char* as_bytes() const { return (const unsigned char*)area(); }
 
-	unsigned char& operator[](size_t index);
-	const unsigned char& operator[](size_t index) const;
+	inline unsigned char& operator[](size_t index) {
+		if(index >= size()) throw "Size too large";
+		return as_bytes()[index];
+	}
+	inline const unsigned char& operator[](size_t index) const {
+		if(index >= size()) throw "Size too large";
+		return as_bytes()[index];
+	}
 
 	static Cow from_raw(cow_t* owned);
 

src/cow.cpp

@@ -39,6 +39,8 @@ Cow Cow::from_raw(cow_t* owned) { if(cow_is_fake(owned)) throw "Trying to create
 Cow::Fake Cow::clone() const { return Fake::from_real(*this); }
 cow_t* Cow::get_raw() const { return super->ptr(); }
 
+size_t Cow::size() const { return super->cow.size; }
+
 Cow::Fake::Fake(const Cow& copy) : Cow(copy), fake(cow_clone(copy.super->ptr())){}
 Cow::Fake::Fake(const Fake& copy) : Cow(copy), fake(cow_clone(copy.fake)){}//Fake(*static_cast<const Cow*>(&copy)){}
 Cow::Fake::Fake(Fake&& move) : Cow(std::move(move)), fake(move.fake)
@@ -54,11 +56,3 @@ cow_t* Cow::Fake::get_raw() const { return fake; }
 
 // Operators
 
-unsigned char& Cow::operator[](size_t index) {
-	if(index >= size()) throw "Size too large";
-	return as_bytes()[index];
-}
-const unsigned char& Cow::operator[](size_t index) const {
-	if(index >= size()) throw "Size too large";
-	return as_bytes()[index];
-}

src/cow_t.h

@@ -16,8 +16,8 @@ extern "C" {
 struct cow_mapped_slice {
 	void* origin; // ptr to mapped memory. This *MUST* be the first field and have an offset of 0.
 
+	size_t size; // Should be at this offset.
 	int fd; // Will be ORd with ~INT_MAX if it's a clone. Will be >0 if it's the original.
-	size_t size;
 }; // cow_t, *cow
 
 #ifdef __cplusplus
@@ -27,6 +27,15 @@ _Static_assert
 #endif
 		(offsetof(cow_t, origin) == 0, "`cow_t.origin` must have an offset of 0.");
 
+#ifndef _COW_NO_ASSUME_ABI
+#ifdef __cplusplus
+static_assert
+#else
+_Static_assert
+#endif
+		(offsetof(cow_t, size) == sizeof(void*), "`cow_t.size` should have an offset equal to `sizeof(void*)` or cow_size_unsafe() becomes UB.");
+#endif
+
 cow_t _cow_create_unboxed(size_t size) internal;
 void _cow_free_unboxed(const cow_t* cow) internal;
 

src/test/main.cpp

@@ -1,9 +1,13 @@
 #include <cow.hpp>
 
+#include <cstdio>
+
 int main()
 {
 	Cow real(4096);
 
+	printf("Fast size: %lu, slow size: %lu\n", real.size_unsafe(), real.size());
+
 	Cow::Fake clone = real;
 	{
 		Cow::Fake clone2 = clone;