3DSoftwareRenderer-DFSPR/Source/DFPSR/base/virtualStack.h

// zlib open source license
//
// Copyright (c) 2024 David Forsgren Piuva
// 
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
// 
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
// 
//    1. The origin of this software must not be misrepresented; you must not
//    claim that you wrote the original software. If you use this software
//    in a product, an acknowledgment in the product documentation would be
//    appreciated but is not required.
// 
//    2. Altered source versions must be plainly marked as such, and must not be
//    misrepresented as being the original software.
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//    3. This notice may not be removed or altered from any source
//    distribution.


#ifndef DFPSR_VIRTUAL_STACK
#define DFPSR_VIRTUAL_STACK

#include "SafePointer.h"

namespace dsr {
	template <typename T>
	constexpr uint64_t memory_getPaddedSize() {
		uint64_t size = (uint64_t)sizeof(T);
		uint64_t alignment = (uint64_t)alignof(T);
		// Round up with unsigned integers.
		return size + (alignment - 1) - ((size - 1) % alignment);
	}

	// Pre-condition:
	//   alignment is a power of two (1, 2, 4, 8, 16, 32, 64...)
	// Post-condition:
	//   Returns a bit mask for rounding an integer down to the closest multiple of alignment.
	constexpr uintptr_t memory_createAlignmentAndMask(uintptr_t alignment) {
		// alignment = ...00001000...
		// Subtracting one from a power of two gives a mask with ones for the remainder bits.
		// remainder = ...00000111...
		// Then we simply negate the mask to get the alignment mask for rounding down.
		// mask      = ...11111000...
		return ~(alignment - 1);
	}

	struct UnsafeAllocation {
		uint8_t *data;
		AllocationHeader *header;
		UnsafeAllocation(uint8_t *data, AllocationHeader *header)
		: data(data), header(header) {}
	};
	// Allocate memory in the virtual stack owned by the current thread.
	//   paddedSize is the number of bytes to allocate including all elements and internal padding.
	//   alignmentMask should only contain zeroes at the bits to round away for alignment.
	UnsafeAllocation virtualStack_push(uint64_t paddedSize, uintptr_t alignmentAndMask);

	// A simpler way to get the correct alignment is to allocate a number of elements with a specific type.
	// TODO: Create another function for manual alignment exceeding the type's alignment using another template argument.
	// TODO: Let the address offset be negated and start with the allocation size going down to zero,
	//       so that rounding up addresses can be done by simply masking the least significant bits.
	template <typename T>
	SafePointer<T> virtualStack_push(uint64_t elementCount, const char *name) {
		// Calculate element size and multiply by element count to get the total size.
		uint64_t paddedSize = memory_getPaddedSize<T>() * elementCount;
		// Allocate the data with the amount of alignment requested by the element type T.
		UnsafeAllocation result = virtualStack_push(paddedSize, memory_createAlignmentAndMask((uintptr_t)alignof(T)));
		// Return a safe pointer to the allocated data.
		return SafePointer<T>(name, (T*)(result.data), (intptr_t)paddedSize, result.header);
	}

	// Free the last allocation from the virtual stack.
	//   Must be called from the same thread that pushed, because virtual stacks are local to their threads.
	void virtualStack_pop();

	// Allocate this array on the stack to automatically free the memory when the scope ends.
	//   Replaces VLA or alloca.
	template <typename T>
	class VirtualStackAllocation : public SafePointer<T> {
	public:
		VirtualStackAllocation(uint64_t elementCount)
		: SafePointer<T>(virtualStack_push<T>(elementCount, "virtual stack allocation")) {}
		~VirtualStackAllocation() {
			virtualStack_pop();
		}
	};
}

#endif