Don't panic when instructions aren't in ISLE (#9631)

Instead return a codegen error which can be more easily caught,
displayed, and rendered. This is intended to help with enabling testing
of Pulley in the near future where Pulley doesn't implement most
instructions at this time and this should make the in-progress state of
the backend a bit easier to test.

Author: alexcrichton

cranelift/codegen/src/machinst/lower.rs

@@ -20,7 +20,7 @@ use crate::machinst::{
     VCodeConstants, VCodeInst, ValueRegs, Writable,
 };
 use crate::settings::Flags;
-use crate::{trace, CodegenResult};
+use crate::{trace, CodegenError, CodegenResult};
 use alloc::vec::Vec;
 use cranelift_control::ControlPlane;
 use rustc_hash::{FxHashMap, FxHashSet};
@@ -676,18 +676,21 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
             // or any of its outputs is used.
             if has_side_effect || value_needed {
                 trace!("lowering: inst {}: {}", inst, self.f.dfg.display_inst(inst));
-                let temp_regs = backend.lower(self, inst).unwrap_or_else(|| {
-                    let ty = if self.num_outputs(inst) > 0 {
-                        Some(self.output_ty(inst, 0))
-                    } else {
-                        None
-                    };
-                    panic!(
-                        "should be implemented in ISLE: inst = `{}`, type = `{:?}`",
-                        self.f.dfg.display_inst(inst),
-                        ty
-                    )
-                });
+                let temp_regs = match backend.lower(self, inst) {
+                    Some(regs) => regs,
+                    None => {
+                        let ty = if self.num_outputs(inst) > 0 {
+                            Some(self.output_ty(inst, 0))
+                        } else {
+                            None
+                        };
+                        return Err(CodegenError::Unsupported(format!(
+                            "should be implemented in ISLE: inst = `{}`, type = `{:?}`",
+                            self.f.dfg.display_inst(inst),
+                            ty
+                        )));
+                    }
+                };
 
                 // The ISLE generated code emits its own registers to define the
                 // instruction's lowered values in. However, other instructions