compiler, runtime: allow slice to array pointer conversion

Panic if the slice is too short.

For golang/go#395

Change-Id: I184f87d0207dcee4be6b36ae446b84e9583b356d
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/338630
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

go/expressions.cc

@@ -3866,11 +3866,12 @@ Type_conversion_expression::do_traverse(Traverse* traverse)
   return TRAVERSE_CONTINUE;
 }
 
-// Convert to a constant at lowering time.
+// Convert to a constant at lowering time.  Also lower conversions
+// from slice to pointer-to-array, as they can panic.
 
 Expression*
 Type_conversion_expression::do_lower(Gogo*, Named_object*,
-				     Statement_inserter*, int)
+				     Statement_inserter* inserter, int)
 {
   Type* type = this->type_;
   Expression* val = this->expr_;
@@ -3958,6 +3959,54 @@ Type_conversion_expression::do_lower(Gogo*, Named_object*,
 	}
     }
 
+  if (type->points_to() != NULL
+      && type->points_to()->array_type() != NULL
+      && !type->points_to()->is_slice_type()
+      && val->type()->is_slice_type())
+    {
+      Temporary_statement* val_temp = NULL;
+      if (!val->is_multi_eval_safe())
+	{
+	  val_temp = Statement::make_temporary(val->type(), NULL, location);
+	  inserter->insert(val_temp);
+	  val = Expression::make_set_and_use_temporary(val_temp, val,
+						       location);
+	}
+
+      Type* int_type = Type::lookup_integer_type("int");
+      Temporary_statement* vallen_temp =
+	Statement::make_temporary(int_type, NULL, location);
+      inserter->insert(vallen_temp);
+
+      Expression* arrlen = type->points_to()->array_type()->length();
+      Expression* vallen =
+	Expression::make_slice_info(val, Expression::SLICE_INFO_LENGTH,
+				    location);
+      vallen = Expression::make_set_and_use_temporary(vallen_temp, vallen,
+						      location);
+      Expression* cond = Expression::make_binary(OPERATOR_GT, arrlen, vallen,
+						 location);
+
+      vallen = Expression::make_temporary_reference(vallen_temp, location);
+      Expression* panic = Runtime::make_call(Runtime::PANIC_SLICE_CONVERT,
+					     location, 2, arrlen, vallen);
+
+      Expression* nil = Expression::make_nil(location);
+      Expression* check = Expression::make_conditional(cond, panic, nil,
+						       location);
+
+      if (val_temp == NULL)
+	val = val->copy();
+      else
+	val = Expression::make_temporary_reference(val_temp, location);
+      Expression* ptr =
+	Expression::make_slice_info(val, Expression::SLICE_INFO_VALUE_POINTER,
+				    location);
+      ptr = Expression::make_unsafe_cast(type, ptr, location);
+
+      return Expression::make_compound(check, ptr, location);
+    }
+
   return this;
 }
 

go/runtime.def

@@ -582,6 +582,11 @@ DEF_GO_RUNTIME(PANIC_EXTEND_SLICE3_C, "runtime.goPanicExtendSlice3C",
 DEF_GO_RUNTIME(PANIC_EXTEND_SLICE3_C_U, "runtime.goPanicExtendSlice3CU",
 	       P2(UINT64, INT), R0())
 
+// Panic for conversion of slice to pointer-to-array if the slice is
+// too short.
+DEF_GO_RUNTIME(PANIC_SLICE_CONVERT, "runtime.goPanicSliceConvert",
+	       P2(INT, INT), R0())
+
 // Remove helper macros.
 #undef ABFT6
 #undef ABFT2

go/types.cc

@@ -842,6 +842,13 @@ Type::are_convertible(const Type* lhs, const Type* rhs, std::string* reason)
 	return true;
     }
 
+  // A slice may be converted to a pointer-to-array.
+  if (rhs->is_slice_type()
+      && lhs->points_to() != NULL
+      && lhs->points_to()->array_type() != NULL
+      && !lhs->points_to()->is_slice_type())
+    return true;
+
   // An unsafe.Pointer type may be converted to any pointer type or to
   // a type whose underlying type is uintptr, and vice-versa.
   if (lhs->is_unsafe_pointer_type()

libgo/go/runtime/error.go

@@ -175,6 +175,7 @@ const (
 	boundsSlice3B    // s[?:x:y], 0 <= x <= y failed (but boundsSlice3A didn't happen)
 	boundsSlice3C    // s[x:y:?], 0 <= x <= y failed (but boundsSlice3A/B didn't happen)
 
+	boundsConvert // (*[x]T)(s), 0 <= x <= len(s) failed
 	// Note: in the above, len(s) and cap(s) are stored in y
 )
 
@@ -190,6 +191,7 @@ var boundsErrorFmts = [...]string{
 	boundsSlice3Acap: "slice bounds out of range [::%x] with capacity %y",
 	boundsSlice3B:    "slice bounds out of range [:%x:%y]",
 	boundsSlice3C:    "slice bounds out of range [%x:%y:]",
+	boundsConvert:    "cannot convert slice with length %y to pointer to array with length %x",
 }
 
 // boundsNegErrorFmts are overriding formats if x is negative. In this case there's no need to report y.

libgo/go/runtime/panic.go

@@ -38,6 +38,7 @@ import (
 //go:linkname goPanicSlice3BU
 //go:linkname goPanicSlice3C
 //go:linkname goPanicSlice3CU
+//go:linkname goPanicSliceConvert
 //go:linkname panicshift
 //go:linkname panicdivide
 //go:linkname panicmem
@@ -175,6 +176,12 @@ func goPanicSlice3CU(x uint, y int) {
 	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3C})
 }
 
+// failures in the conversion (*[x]T)s, 0 <= x <= y, x == cap(s)
+func goPanicSliceConvert(x int, y int) {
+	panicCheck1(getcallerpc(), "slice length too short to convert to pointer to array")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsConvert})
+}
+
 var shiftError = error(errorString("negative shift amount"))
 
 func panicshift() {