SIMD-0166: SBPF Dynamic stack frames

proposals/0166-dynamic-stack-frames.md

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+---
+simd: '0166'
+title: SBPF Dynamic stack frames
+authors:
+  - Alexander Meißner
+  - Alessandro Decina
+  - Lucas Steuernagel
+category: Standard
+type: Core
+status: Review
+created: 2024-08-19T00:00:00.000Z
+feature: null
+supersedes: null
+superseded-by: null
+extends: null
+---
+
+## Summary
+
+The SVM currently allocates a fixed amount of stack space to each function 
+frame. We propose allowing programs to dynamically manage their stack space 
+through the introduction of an explicit stack pointer.
+
+## Motivation
+
+The SVM allocates a fixed amount of memory to hold a program’s stack. Within 
+the stack region, the virtual machine reserves 4096 bytes of stack space for 
+each function frame. This is simultaneously limiting for functions that 
+require more space, and wasteful for functions that require less space.
+
+For well optimized programs that don’t allocate large amounts of stack, the 
+virtual machine currently still reserves 4096 bytes of stack for each function 
+call, leading to suboptimal memory usage, which may cause unnecessary page 
+faults.
+
+On the other hand, some programs are known to create large function frames - 
+this seems common with programs that serialize a lot of data - and they have 
+to jump through hoops to avoid overflowing the stack. The virtual machine 
+detects when a stack overflow occurs, and it does so by implementing a stack 
+frame gaps system whereby it inserts a virtual sentinel frame following a 
+valid function frame. If the sentinel frame is accessed, the executing program 
+is aborted. This system is fragile and is incompatible with direct mapping - a 
+feature we expect to enable soon.
+
+The changes proposed in this document would allow us to optimize stack memory 
+usage and remove the fragile stack frame gaps system. Note that we do not 
+propose to remove the existing maximum stack space limit: stack space stays 
+unchanged, what changes is how it is partitioned internally.
+
+## Alternatives Considered
+
+To cope with the SBF limitation of 4096 bytes for the frame size, we could 
+have increased such a number. Even though this would solve the original 
+problem, it would supply functions with an unnecessary amount of memory. In 
+addition, such a solution would increase pressure on the total memory 
+available for the call stack. Either we would need to increase the total 
+allocation for the virtual machine or decrease the maximum call depth.
+
+## New Terminology
+
+None.
+
+## Detailed Design
+
+Bringing dynamic stack frames to the Solana Bytecode Format and its 
+corresponding virtual machine entails changes in several aspects of the 
+execution environment.
+
+### Changes in the execution environment
+
+We will repurpose the existing R10 register from a frame pointer to a stack 
+pointer. In other words, it must stop representing the highest address 
+accessible in a frame, and must now point to the lowest address in a frame.
+
+Such a change entails a change in the direction of stack growth. Presently, we 
+stack frames on top of each other, but the memory usage within them grows 
+downward. In the new frame setting, both the placement of new frames and the 
+memory usage inside frames must be downward.
+
+Functions in SBF must alter the stack pointer using the `add64 reg, imm` 
+(opcode `0x07`) instruction only, allowing them to request any desirable 
+amount of stack space, provided that it meets the required alignment (refer to 
+the following section).
+
+The stack frame gaps feature, which creates a memory layout where frames are 
+interleaved with equally sized gaps, are not compatible with dynamic stack 
+frames and must be deactivated.
+
+### Stack alignment
+
+We want to enforce that the stack pointer remains aligned, therefore R10 must 
+only be incremented or decremented by a multiple of 64. Large alignments might 
+seem wasteful, but enforcing a sufficiently big alignment will spark 
+innovation in interpreters and JITs, ultimately leading to much better 
+performance and thus lower costs.
+
+Based on the current AVX-512 instructions available on Intel and AMD 
+processors, the stack alignment must be 64 bytes. Even if current interpreters 
+do not take advantage of these vectorized instructions, we believe that future 
+generation interpreters might be able to vectorize SBF programs to speed up 
+common operations, such as copying or comparing public keys and signatures. 
+An unaligned stack prohibits such innovations.
+
+### Changes in the verifier
+
+The verifier must now allow R10 to be the destination register of the 
+`add64 reg, imm` (opcode `0x07`) instruction.
+
+The verifier must throw `VerifierError::UnalignedImmediate` when the immediate 
+value of `add64 reg, imm` (opcode `0x07`) is not a multiple of 64 and the 
+destination register is R10. The error must only be raised when both 
+conditions happen simultaneously.
+
+### Changes in code generation
+
+In the compiler side, dynamic stack frames allow for some optimizations. 
+First, when a function does not need any stack allocated variable, code 
+generation must not create any instruction to modify R10. In addition, we can 
+stop using R5 as a stack spill register when a function call receives more 
+than five arguments. With dynamic stack frames, the compiler must use 
+registers R1 to R5 for the first five arguments and place remainder arguments 
+in the caller frame, easily retrieving them in the callee as an offset from 
+the stack pointer. This new call convention obviates the need to use R5 for 
+retrieving the caller’s frame pointer address to access those parameters.
+
+### Identification of programs
+
+As per the description in SIMD-0161, programs compiled with dynamic stack 
+frames must contain the `0x02` flag on their ELF header `e_flags` field.
+
+## Impact
+
+We foresee a positive impact in smart contract development. Developers won’t 
+need to worry about exceeding the maximum frame space allowed for a function 
+and won’t face any case of stack access violation if their code follows 
+conventional Rust safety rules. Likewise, when we update the Rust version of 
+our platform tools, developers will not have the burden of modifying their 
+contract just because the newer version is using more stack than the previous 
+one, often reaching the 4096 bytes limit. Refer to issues 
+[#1186](https://github.com/anza-xyz/agave/issues/1186) and 
+[#1158](https://github.com/anza-xyz/agave/issues/1158).
+
+We also expect some improvements in program execution. For functions with no 
+stack usage, we will not emit the additional instruction that modifies R10. 
+Furthermore, for function calls that handle more than five arguments, there 
+will be one less store and one less load operation due to the new call convention.
+
+## Security Considerations
+
+Stack gaps will be disabled for dynamic stack frames to work. Stack gaps could 
+detect invalid accesses between two function frames, if the accessed address 
+would fall between them. With dynamic stack frames, all stack access will be 
+valid, provided that their address is within the allowed range. We already 
+allow functions to read and modify the memory inside the frame of other 
+functions, so removing the stack gaps should not bring any security 
+implications.
+
+Although one can change R10 to almost any value that fits in a 64-bit integer 
+with `add64 reg, imm`, every memory access is verified, so there is no risk of 
+invalid accesses from a corrupt register.
+
+## Drawbacks
+
+Programs will consume negligibly more compute units, as most functions will 
+include two extra instructions: one to increment the stack pointer and another 
+one to decrement it.