[Strings] Remove stringview types and instructions (#6579)

The stringview types from the stringref proposal have three irregularities that
break common invariants and require pervasive special casing to handle properly:
they are supertypes of `none` but not subtypes of `any`, they cannot be the
targets of casts, and they cannot be used to construct nullable references. At
the same time, the stringref proposal has been superseded by the imported
strings proposal, which does not have these irregularities. The cost of
maintaing and improving our support for stringview types is no longer worth the
benefit of supporting them.

Simplify the code base by entirely removing the stringview types and related
instructions that do not have analogues in the imported strings proposal and do
not make sense in the absense of stringviews.

Three remaining instructions, `stringview_wtf16.get_codeunit`,
`stringview_wtf16.slice`, and `stringview_wtf16.length` take stringview operands
in the stringref proposal but cannot be removed because they lower to operations
from the imported strings proposal. These instructions are changed to take
stringref operands in Binaryen IR, and to allow a graceful upgrade path for
users of these instructions, the text and binary parsers still accept but ignore
`string.as_wtf16`, which is the instruction used to convert stringrefs to
stringviews. The binary writer emits code sequences that use scratch locals and `string.as_wtf16` to keep the output valid.

Future PRs will further align binaryen with the imported strings proposal
instead of the stringref proposal, for example by making `string` a subtype of
`extern` instead of a subtype of `any` and by removing additional instructions
that do not have analogues in the imported strings proposal.

1 files changed, 1 insertions, 38 deletions

diff --git a/src/wasm-interpreter.h b/src/wasm-interpreter.h
index ec1ba7707..bd7d78a6e 100644
--- a/src/wasm-interpreter.h
+++ b/src/wasm-interpreter.h
@@ -1919,7 +1919,7 @@ public:
 
   Flow visitStringMeasure(StringMeasure* curr) {
     // For now we only support JS-style strings.
-    if (curr->op != StringMeasureWTF16View && curr->op != StringMeasureWTF16) {
+    if (curr->op != StringMeasureWTF16) {
       return Flow(NONCONSTANT_FLOW);
     }
 
@@ -2076,29 +2076,6 @@ public:
     }
     return Literal(result);
   }
-  Flow visitStringAs(StringAs* curr) {
-    // For now we only support JS-style strings.
-    if (curr->op != StringAsWTF16) {
-      return Flow(NONCONSTANT_FLOW);
-    }
-
-    Flow flow = visit(curr->ref);
-    if (flow.breaking()) {
-      return flow;
-    }
-    auto value = flow.getSingleValue();
-    auto data = value.getGCData();
-    if (!data) {
-      trap("null ref");
-    }
-
-    // A JS-style string can be viewed simply as the underlying data. All we
-    // need to do is fix up the type.
-    return Literal(data, curr->type.getHeapType());
-  }
-  Flow visitStringWTF8Advance(StringWTF8Advance* curr) {
-    return Flow(NONCONSTANT_FLOW);
-  }
   Flow visitStringWTF16Get(StringWTF16Get* curr) {
     NOTE_ENTER("StringWTF16Get");
     Flow ref = visit(curr->ref);
@@ -2122,18 +2099,7 @@ public:
 
     return Literal(values[i].geti32());
   }
-  Flow visitStringIterNext(StringIterNext* curr) {
-    return Flow(NONCONSTANT_FLOW);
-  }
-  Flow visitStringIterMove(StringIterMove* curr) {
-    return Flow(NONCONSTANT_FLOW);
-  }
   Flow visitStringSliceWTF(StringSliceWTF* curr) {
-    // For now we only support JS-style strings.
-    if (curr->op != StringSliceWTF16) {
-      return Flow(NONCONSTANT_FLOW);
-    }
-
     Flow ref = visit(curr->ref);
     if (ref.breaking()) {
       return ref;
@@ -2167,9 +2133,6 @@ public:
     }
     return makeGCData(contents, curr->type);
   }
-  Flow visitStringSliceIter(StringSliceIter* curr) {
-    return Flow(NONCONSTANT_FLOW);
-  }
 
   virtual void trap(const char* why) { WASM_UNREACHABLE("unimp"); }