SIMD-0194: Deprecate rent exemption threshold

proposals/XXXX-deprecate-rent-exemption-threshold.md

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+---
+simd: '0XXX'
+title: Deprecate Rent Exemption Threshold
+authors:
+  - Dean Little (@deanmlittle)
+  - Leonardo Donatacci (@L0STE)
+  - febo (Anza)
+category: Standard/Meta
+type: Core
+status: Draft
+created: 2024-11-13
+feature: (fill in with feature tracking issues once accepted)
+---
+
+## Summary
+
+This proposal aims to eliminate the use of floating-point operations to 
+determine whether an account is rent exempt or not in programs by deprecating
+the use of the `exempt_threshold` (`f64`) value. 
+
+More specifically, rename `lamports_per_byte_year` to `lamports_per_byte`, 
+change default value from `3480` to `6960`, change `exemption_threshold` to 
+`1.0` and deprecate it from the protocol. This will enable us to remove `f64` 
+math from all Rent-related SDKs and onchain programs moving forward.
+
+## Motivation
+
+Emulating floating point math is expensive inside of SVM due to the lack of 
+native floating point number support. This makes calculating the rent exempt 
+threshold cost `~248` more CUs than it would if we were to simply use a `u64`. 
+This is due to the `exemption_threshold` (`f64`), which is currently set to 
+`2.0` years. Since rent exemption is no longer time-based, we have the 
+opportunity to make this commonly used calculation much more performant and 
+simplify our tooling without affecting the existing API. It also simplifies any 
+further changes we may want to make to Rent down the line.
+
+## New Terminology
+
+`lamports_per_byte` - the number of lamports required to pay for 1 byte of 
+account storage.
+
+## Detailed Design
+
+Set the value of `DEFAULT_EXEMPTION_THRESHOLD` from `2.0` to `1.0`, and 
+deprecate it from the protocol.
+
+```rs
+pub const DEFAULT_EXEMPTION_THRESHOLD: f64 = 1.0;
+```
+
+Set `DEFAULT_LAMPORTS_PER_BYTE` from its current value of `3480` (the `u64` 
+value of `1_000_000_000 / 100 * 365 / (1024 * 1024)`), to `6960`; double its 
+current `u64` value, to counteract reducing the exemption threshold above.
+
+```rs
+pub const DEFAULT_LAMPORTS_PER_BYTE: u64 = 6960;
+```
+
+Rename `lamports_per_byte_year` in the Rent account to `lamports_per_byte` to 
+reflect that Rent is no longer time-based and officially deprecate 
+`exemption_threshold`.
+
+```rs
+pub struct Rent {
+    pub lamports_per_byte: u64,
+    #[deprecated(since = "2.X.X", note = "Use only `lamports_per_byte` 
+instead")]
+    pub exemption_threshold: f64,
+    pub burn_percent: u8,
+}
+```
+
+ Remove all `f64` math from Rent calculations in all SDKs and onchain programs 
+moving forwards, replacing it with simple `u64` math, ie:
+
+```rs
+/// Minimum balance due for rent-exemption of a given account data size.
+pub fn minimum_balance(&self, data_len: usize) -> u64 {
+    (ACCOUNT_STORAGE_OVERHEAD + data_len as u64) * self.lamports_per_byte
+}
+```
+
+Validator implementations should stop using `exemption_threshold` and only use
+the `lamports_per_byte` value. Any existing program using the current 
+implementation will be unaffected.
+
+## Alternatives Considered
+
+* Leave things as they are.
+  - Calculating rent exemption on a program remains an 
+expensive operation.
+
+* Allow users to make the assumption that `2.0` will remain stable and do `u64` 
+math themselves.
+  - Risk of the protocol changing on them.
+
+* Perform the "conversion" ouside the VM and change the type of 
+`exemption_threshold` to a `u64` using a new `RentV2` struct.
+  - Requires a new syscall and sysvar.
+
+## Impact
+
+This change will significantly improve the compute unit (CU) consumption of
+new onchain programs using updated SDKs programs that calculate the minimum 
+lamports balance required for rent exemption. Currently, the calculation of the 
+minimum balance for rent exemption (`Rent::minimum_balance`) consumes
+approximately `256` CUs; with the proposed change, this will be reduced to
+just `8` CUs. Existing programs will not be impacted.
+
+## Security Considerations
+
+None.
+
+## Drawbacks
+
+None.
+
+## Backwards Compatibility
+
+This feature is backwards compatible. It does not change the absolute value 
+required for an account to be rent exempt and the current way of calculating 
+the threshold will continue to evaluate to the same value. Any deployed program 
+will be unaffected.