Merge pull request #64 from bendyarm/flagged-operations

Flagged operations

arc-0004/README.md

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+---
+arc: 4
+title: Flagged Operations
+authors: bendyarm, d0cd, acoglio
+discussion: https://github.com/AleoHQ/ARCs/discussions/63
+topic: # Choose: Protocol, Network, or Application
+status: Draft
+created: # Date
+---
+
+## Abstract
+
+Some Aleo instructions can halt when passed certain arguments.
+This halting behavior makes those instructions unusable
+by a program that wants to try something else if a halt is detected.
+
+For example, if a Leo program contains a conditional not in a
+finalize, both branches of the conditional are executed in the
+circuit, so if a branch not taken executes an instruction that halts,
+the whole program will incorrectly halt.  Replacing the halting
+instruction by one that returns an error flag will allow such a
+program to compile correctly.
+
+This ARC proposes to add new opcodes for flagged operations
+corresponding to the Aleo Instructions that can halt.
+
+## Specification
+
+Each flagged operation is identical to the current halting instruction
+except it doesn't halt and it returns another return value that is a
+boolean.
+
+The flagged operations are different from wrapped (e.g. `abs.w`) or
+lossy (e.g. `cast.lossy`) operations.  It is important that the flagged
+operation have the same semantics as the current halting instruction
+except for the halting behavior and extra return value, for ease of
+use by compilers.
+
+| Current Halting Opcode | New Flagged Opcode |
+|:-------------------:|:-----------------------:|
+| abs | abs.flagged |
+| add | add.flagged |
+| cast | cast.flagged |
+| commit.bhp256 | commit.bhp256.flagged |
+| commit.bhp512 | commit.bhp512.flagged |
+| commit.bhp768 | commit.bhp768.flagged |
+| commit.bhp768 | commit.bhp1024.flagged |
+| commit.ped64 | commit.ped64.flagged |
+| commit.ped128 | commit.ped128.flagged |
+| div | div.flagged |
+| hash.bhp256 | hash.bhp256.flagged |
+| hash.bhp512 | hash.bhp512.flagged |
+| hash.bhp768 | hash.bhp768.flagged |
+| hash.bhp1024 | hash.bhp1024.flagged |
+| hash.ped64 | hash.ped64.flagged |
+| hash.ped128 | hash.ped128.flagged |
+| inv | inv.flagged |
+| mod | mod.flagged |
+| mul | mul.flagged |
+| neg | neg.flagged |
+| pow | pow.flagged |
+| rem | rem.flagged |
+| shl | shl.flagged |
+| shr | shr.flagged |
+| sqrt | sqrt.flagged |
+| sub | sub.flagged |
+
+### Test Cases
+
+This section should introduce any and all critical test cases that need to be considered for the specification.
+
+The following note applies to all tests of Aleo Instructions.  For all tests, both literal constant arguments
+and variable arguments should be tested.  If there are two arguments, we need to test all four
+combinations.  This is because the Aleo Instructions compiler generates more optimized code for literal
+constant arguments, and the circuits can differ substantially.
+
+One or more arguments that causes halting for a current halting opcode should be used as an input
+for the equivalent new flagged opcode to make sure it doesn't halt.  
+
+An assortment of arguments that do not cause halting for a current halting opcode should be
+used as input to both halting and flagged operations to make sure they return the same value
+(other than the halting flag, of course).
+
+## Dependencies
+
+The main change is to the snarkVM repository.
+
+The Aleo Explorer will need to be updated to output the new operations when displaying an aleo program.
+
+The "sdk" repository may need to be changed to show syntax highlighting on the new opcodes.
+
+After this change, the Leo compiler can more easily be fixed to prevent the [conditional halting bug.](<https://github.com/AleoHQ/leo/issues/27482>)
+
+### Backwards Compatibility
+
+This change is strictly additive.
+
+## Security & Compliance
+
+This change does not affect security or regulatory issues.
+
+## References
+
+Leo bug: 
+https://github.com/AleoHQ/leo/issues/27482