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Remove leftover arithmetic overflow checks
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117 changes: 117 additions & 0 deletions
117
charon/src/transform/remove_arithmetic_overflow_checks.rs
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,117 @@ | ||
| //! # Micro-pass: remove the overflow checks for arithmetic operations we couldn't remove in | ||
| //! [`remove_dynamic_checks`]. See comments there for more details. | ||
| use crate::formatter::{Formatter, IntoFormatter}; | ||
| use crate::llbc_ast::*; | ||
| use crate::translate_ctx::TransCtx; | ||
| use crate::types::*; | ||
| use take_mut::take; | ||
|
|
||
| struct RemoveDynChecks; | ||
|
|
||
| impl RemoveDynChecks { | ||
| /// After `remove_dynamic_checks`, the only remaining dynamic checks are overflow checks. We | ||
| /// couldn't remove them in ullbc because the generated code spans two basic blocks. They are | ||
| /// inserted only in constants since we otherwise compile in release mode. These assertions | ||
| /// look like: | ||
| /// ```text | ||
| /// r := x checked.+ y; | ||
| /// assert(move r.1 == false); | ||
| /// z := move r.0; | ||
| /// ``` | ||
| /// We replace that with: | ||
| /// ```text | ||
| /// z := x + y; | ||
| /// ``` | ||
| fn simplify(&mut self, s: &mut Statement) { | ||
| if let RawStatement::Sequence(s0, s1) = &mut s.content { | ||
| if let RawStatement::Sequence(s1, s2) = &mut s1.content { | ||
| // TODO: the last statement is not necessarily a sequence | ||
| if let RawStatement::Sequence(s2, _) = &mut s2.content { | ||
| if let ( | ||
| RawStatement::Assign( | ||
| binop, | ||
| Rvalue::BinaryOp( | ||
| op @ (BinOp::CheckedAdd | BinOp::CheckedSub | BinOp::CheckedMul), | ||
| _, | ||
| _, | ||
| ), | ||
| ), | ||
| RawStatement::Assert(Assert { | ||
| cond: Operand::Move(assert_cond), | ||
| expected: false, | ||
| }), | ||
| RawStatement::Assign(final_value, Rvalue::Use(Operand::Move(assigned))), | ||
| ) = (&mut s0.content, &s1.content, &mut s2.content) | ||
| { | ||
| // assigned should be: binop.0 | ||
| // assert_cond should be: binop.1 | ||
| if let ( | ||
| [ProjectionElem::Field(FieldProjKind::Tuple(..), fid0)], | ||
| [ProjectionElem::Field(FieldProjKind::Tuple(..), fid1)], | ||
| ) = ( | ||
| assigned.projection.as_slice(), | ||
| assert_cond.projection.as_slice(), | ||
| ) { | ||
| if assert_cond.var_id == binop.var_id | ||
| && assigned.var_id == binop.var_id | ||
| && binop.projection.len() == 0 | ||
| && fid0.index() == 0 | ||
| && fid1.index() == 1 | ||
| { | ||
| // Switch to the unchecked operation. | ||
| *op = match op { | ||
| BinOp::CheckedAdd => BinOp::Add, | ||
| BinOp::CheckedSub => BinOp::Sub, | ||
| BinOp::CheckedMul => BinOp::Mul, | ||
| _ => unreachable!(), | ||
| }; | ||
| // Assign to the correct value in `s0`. | ||
| std::mem::swap(binop, final_value); | ||
| // Remove `s1` and `s2`. | ||
| take(s, |s| { | ||
| let (s0, s1) = s.content.to_sequence(); | ||
| let (_s1, s2) = s1.content.to_sequence(); | ||
| let (_s2, s3) = s2.content.to_sequence(); | ||
| Statement { | ||
| meta: s0.meta, | ||
| content: RawStatement::Sequence(s0, s3), | ||
| } | ||
| }); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| impl MutTypeVisitor for RemoveDynChecks {} | ||
| impl MutExprVisitor for RemoveDynChecks {} | ||
|
|
||
| impl MutAstVisitor for RemoveDynChecks { | ||
| fn spawn(&mut self, visitor: &mut dyn FnMut(&mut Self)) { | ||
| visitor(self) | ||
| } | ||
|
|
||
| fn merge(&mut self) {} | ||
|
|
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| fn visit_statement(&mut self, s: &mut Statement) { | ||
| // Simplify this statement. | ||
| self.simplify(s); | ||
| // Recurse into subsequent statements. | ||
| self.default_visit_raw_statement(&mut s.content); | ||
| } | ||
| } | ||
|
|
||
| pub fn transform(ctx: &mut TransCtx, funs: &mut FunDecls, globals: &mut GlobalDecls) { | ||
| ctx.iter_bodies(funs, globals, |ctx, name, b| { | ||
| let fmt_ctx = ctx.into_fmt(); | ||
| trace!( | ||
| "# About to remove the remaining overflow checks: {}:\n{}", | ||
| name.fmt_with_ctx(&fmt_ctx), | ||
| fmt_ctx.format_object(&*b) | ||
| ); | ||
| RemoveDynChecks.visit_statement(&mut b.body); | ||
| }) | ||
| } |
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