Allow fixed-early-def and fixed-late-use constraints

These are useful to model fixed registers which should not be reused for
other uses/defs. These were disallowed in #54, but this was too
conservative.

Fundamentally, the only situation where a preg can be used in multiple
fixed constraints within a single instruction is with an early-use and a
late-def. Anything else is a user error because the live ranges will
overlap.

As such this PR relaxes the operand rewrite from #54 to only apply in
this specific situation. Fixed-late-use and fixed-early-def operands are
left unchanged.

src/fuzzing/func.rs

@@ -471,29 +471,27 @@ impl Func {
                         let i = u.int_in_range(0..=(operands.len() - 1))?;
                         let op = operands[i];
                         let fixed_reg = PReg::new(u.int_in_range(0..=62)?, op.class());
+                        if op.kind() == OperandKind::Def && op.pos() == OperandPos::Early {
+                            // Early-defs with fixed constraints conflict with
+                            // any other fixed uses of the same preg.
+                            if fixed_late.contains(&fixed_reg) {
+                                break;
+                            }
+                        }
+                        if op.kind() != OperandKind::Def && op.pos() == OperandPos::Late {
+                            // Late-use with fixed constraints conflict with
+                            // any other fixed uses of the same preg.
+                            if fixed_early.contains(&fixed_reg) {
+                                break;
+                            }
+                        }
                         let fixed_list = match op.pos() {
                             OperandPos::Early => &mut fixed_early,
                             OperandPos::Late => &mut fixed_late,
                         };
                         if fixed_list.contains(&fixed_reg) {
                             break;
                         }
-                        if op.kind() != OperandKind::Def && op.pos() == OperandPos::Late {
-                            // Late-uses/mods with fixed constraints
-                            // can't be allowed if we're allowing
-                            // different constraints at Early and
-                            // Late, because we can't move something
-                            // into a location between Early and
-                            // Late. Differing constraints only make
-                            // sense if the instruction itself
-                            // produces the newly-constrained values.
-                            break;
-                        }
-                        if op.kind() != OperandKind::Use && op.pos() == OperandPos::Early {
-                            // Likewise, we can *only* allow uses for
-                            // fixed constraints at Early.
-                            break;
-                        }
                         fixed_list.push(fixed_reg);
                         operands[i] = Operand::new(
                             op.vreg(),

src/ion/liveranges.rs

@@ -479,23 +479,18 @@ impl<'a, F: Function> Env<'a, F> {
                 // for the use, (ii) rewrite the use to have an Any
                 // constraint, and (ii) move the def to Early position
                 // to reserve the register for the whole instruction.
+                //
+                // We don't touch any fixed-early-def or fixed-late-use
+                // constraints: the only situation where the same physical
+                // register can be used multiple times in the same
+                // instruction is with an early-use and a late-def. Anything
+                // else is a user error.
                 let mut operand_rewrites: FxHashMap<usize, Operand> = FxHashMap::default();
                 let mut late_def_fixed: SmallVec<[PReg; 8]> = smallvec![];
                 for &operand in self.func.inst_operands(inst) {
                     if let OperandConstraint::FixedReg(preg) = operand.constraint() {
-                        match operand.pos() {
-                            OperandPos::Late => {
-                                // See note in fuzzing/func.rs: we
-                                // can't allow this, because there
-                                // would be no way to move a value
-                                // into place for a late use *after*
-                                // the early point (i.e. in the middle
-                                // of the instruction).
-                                assert!(
-                                    operand.kind() == OperandKind::Def,
-                                    "Invalid operand: fixed constraint on Use/Mod at Late point"
-                                );
-
+                        match (operand.pos(), operand.kind()) {
+                            (OperandPos::Late, OperandKind::Def) => {
                                 late_def_fixed.push(preg);
                             }
                             _ => {}
@@ -507,12 +502,11 @@ impl<'a, F: Function> Env<'a, F> {
                         continue;
                     }
                     if let OperandConstraint::FixedReg(preg) = operand.constraint() {
-                        match operand.pos() {
-                            OperandPos::Early if live.get(operand.vreg().vreg()) => {
-                                assert!(operand.kind() == OperandKind::Use,
-                                            "Invalid operand: fixed constraint on Def/Mod at Early position");
-
-                                // If we have a constraint at the
+                        match (operand.pos(), operand.kind()) {
+                            (OperandPos::Early, OperandKind::Use)
+                                if live.get(operand.vreg().vreg()) =>
+                            {
+                                // If we have a use constraint at the
                                 // Early point for a fixed preg, and
                                 // this preg is also constrained with
                                 // a *separate* def at Late or is