Relax verification to allow I8X16 to act as a default vector type

cranelift-codegen/src/verifier/mod.rs

@@ -64,6 +64,7 @@ use crate::flowgraph::{BasicBlock, ControlFlowGraph};
 use crate::ir;
 use crate::ir::entities::AnyEntity;
 use crate::ir::instructions::{BranchInfo, CallInfo, InstructionFormat, ResolvedConstraint};
+use crate::ir::types::I8X16;
 use crate::ir::{
     types, ArgumentLoc, Ebb, FuncRef, Function, GlobalValue, Inst, InstructionData, JumpTable,
     Opcode, SigRef, StackSlot, StackSlotKind, Type, Value, ValueDef, ValueList, ValueLoc,
@@ -1189,6 +1190,12 @@ impl<'a> Verifier<'a> {
             let arg_type = self.func.dfg.value_type(arg);
             match constraints.value_argument_constraint(i, ctrl_type) {
                 ResolvedConstraint::Bound(expected_type) => {
+                    // Skip checking of the V128 default type (i.e. I8X16): this relaxation of type
+                    // checking allows incoming values of I8X16 (say from a function signature) to
+                    // be used as any vector type of the same width.
+                    if arg_type == I8X16 && expected_type.bits() == arg_type.bits() {
+                        continue;
+                    }
                     if arg_type != expected_type {
                         report!(
                             errors,
@@ -1412,6 +1419,15 @@ impl<'a> Verifier<'a> {
             }
             for (i, (&arg, &expected_type)) in args.iter().zip(expected_types).enumerate() {
                 let arg_type = self.func.dfg.value_type(arg);
+                // Skip checking of the V128 default type (i.e. I8X16): this relaxation of type
+                // checking is the inverse of the one in `typecheck_fixed_args` above. It allows
+                // values returned from a function to be any vector of the same width as the
+                // default type.
+                if expected_type.value_type == I8X16
+                    && arg_type.bits() == expected_type.value_type.bits()
+                {
+                    continue;
+                }
                 if arg_type != expected_type.value_type {
                     report!(
                         errors,

filetests/verifier/simd-default-type.clif

@@ -0,0 +1,39 @@
+test verifier
+set enable_simd
+target x86_64
+
+; This file tests using I8X16 as a default type to represent Wasm's V128 type. Values of Wasm's V128
+; type can be used in any instruction of similar width (e.g. `i32x4.add`). Cranelift must build
+; signatures for Wasm functions using V128 and, lacking a special type to do so, Cranelift
+; uses I8X16 for V128. To avoid excessive bitcasting, however, we relax the verifier to allow
+; I8X16 to type-check as the other 128-bit types. This implies that:
+;  - SIMD instructions must have explicit types (e.g. iadd.i32x4) to avoid type inference deciding
+;  a wrong type and emitting the wrong instruction
+;  - The verifier will be unable to catch unintended errors in sequences like: `v1 = iadd.i8x16 v0,
+;  v0; v2 = isub.i32x4 v1, v0` (presumably we want v0 and v1 to be I8X16 in fact and would want the
+;  verifier to tell us that we should not add them as I32X4s).
+
+function %passes_non_default_type(i32x4) {
+ebb0(v0: i32x4):
+    v1 = fneg.f32x4 v0 ; error: arg 0 (v0) has type i32x4, expected f32x4
+    return
+}
+
+function %passes_default_type(i8x16) {
+ebb0(v0: i8x16):
+    v1 = fadd.f32x4 v0, v0
+    return
+}
+
+function %returns_non_default_type() -> i64x2 {
+ebb0:
+    v1 = vconst.i32x4 [0 1 2 3]
+    return v1 ; error: arg 0 (v1) has type i32x4, must match function signature of i64x2
+}
+
+function %returns_default_type() -> i8x16 {
+ebb0:
+    v1 = vconst.i32x4 [0 1 2 3]
+    return v1
+}
+