Merge pull request #1299 from vlstill/string-concat

Allow concatenation of string literals at compile time

Author: jonathan-dilorenzo

p4-16/spec/P4-16-spec.adoc

@@ -1225,11 +1225,18 @@ number of backslash characters (ASCII code 92). P4 does not make any
 validity checks on strings (i.e., it does not check that strings
 represent legal UTF-8 encodings).
 
-Since P4 does not provide any operations on strings,
-string literals are generally passed unchanged through the P4 compiler to
-other third-party tools or compiler-backends, including the
-terminating quotes. These tools can define their own handling of
-escape sequences (e.g., how to specify Unicode characters, or handle
+Since P4 does not allow strings to exist at runtime, string literals
+are generally passed unchanged through the P4 compiler to
+other third-party tools or compiler-backends. The compiler can, however,
+perform compile-time concatenation (constant-folding) of concatenation
+expressions into single literal. When such concatenation is performed,
+the binary representation of the string literals (excluding the quotes)
+is concatenated in the order they appears in the source code. There are
+no escape sequences that would be treated specially when strings are
+concatenated.
+
+The backends and other tools can define their own handling of escape
+sequences (e.g., how to specify Unicode characters, or handle
 unprintable ASCII characters).
 
 Here are 3 examples of string literals:
@@ -1242,6 +1249,17 @@ Here are 3 examples of string literals:
 line terminator"
 ----
 
+Here is an example of concatenation expression and an equivalent string
+literal:
+
+[source,p4]
+----
+"one string \" with a quote inside;" ++ (" " ++ "another string")
+// can be constant folded to
+"one string \" with a quote inside; another string"
+----
+
+
 [#sec-trailing-commas]
 ==== Optional trailing commas
 
@@ -1867,15 +1885,20 @@ Operations on values of type `match_kind` are described in
 ==== The Boolean type
 
 The Boolean type `bool` contains just two values, `false` and `true`.
-Boolean values are not integers or bit-strings.
+Boolean values are not integers or bit-strings. Operations that can
+be performed on booleans are described in Section <<sec-bool-exprs>>.
 
 [#sec-string-type]
 ==== Strings
 
-The type `string` represents strings.  There are no operations on
-string values; one cannot declare variables with a `string` type.
-Parameters with type `string` can be only directionless (see
-<<sec-calling-convention>>).
+The type `string` represents strings. The values of type `string` are
+either string literals, or concatenations of multiple `string`-typed
+expression. Operations that can be performed on strings are described in
+Section <<sec-string-ops>>.
+
+One cannot declare variables with a `string` type. Parameters with
+type `string` can be only directionless (see Section
+<<#sec-calling-convention>>).
 P4 does not support string manipulation
 in the dataplane; the `string` type is only allowed for describing
 compile-time known values (i.e., string literals, as discussed in
@@ -3487,6 +3510,25 @@ itself can be evaluated at compilation time. This restriction is
 designed to ensure that the width of the result of the conditional
 expression can be inferred statically at compile time.
 
+[#sec-string-ops]
+=== Operations on strings
+
+The only operation allowed on strings is concatenation, denoted by
+`++`. For string concatenation, both operands must be strings and
+the result is also a string. String concatenation can only be
+performed at compile time.
+
+[source,p4]
+----
+extern void log(string message);
+
+void foo(int<8> v) {
+  // ...
+  log("my log message " ++
+      "continuation of the log message");
+}
+----
+
 [#sec-bit-ops]
 === Operations on fixed-width bit types (unsigned integers)
 
@@ -8903,9 +8945,10 @@ table t {
 
 The `@name` annotation directs the compiler to use a different
 local name when generating the external APIs used to manipulate a
-language element from the control plane. This annotation takes a string literal
-body.  In the
-following example, the fully-qualified name of the table is `c_inst.t1`.
+language element from the control plane. This annotation takes a local
+compile-time known value of type `string` (typically a string literal).
+In the following example, the fully-qualified name of the table is
+`c_inst.t1`.
 
 [source,p4]
 ----
@@ -9036,12 +9079,14 @@ absence), allowing architecture-independent analysis of P4 programs.
 
 The `deprecated` annotation has a required string argument that is a
 message that will be printed by a compiler when a program is using the
-deprecated construct.  This is mostly useful for annotating library
-constructs, such as externs.
+deprecated construct. This is mostly useful for annotating library
+constructs, such as externs. The parameter must be a local
+compile-time known value of type `string`.
 
 [source,p4]
 ----
-@deprecated("Please use the 'check' function instead")
+#define DEPR_V1_2_2 "Deprecated in v1.2.2"
+@deprecated("Please use the 'check' function instead." ++ DEPR_V1_2_2)
 extern Checker {
    /* body omitted */
 }
@@ -9053,7 +9098,8 @@ extern Checker {
 The `noWarn` annotation has a required string argument that indicates
 a compiler warning that will be inhibited.  For example
 `@noWarn("unused")` on a declaration will prevent a compiler warning
-if that declaration is not used.
+if that declaration is not used. The parameter must be a local
+compile-time known value of type `string`.
 
 === Target-specific annotations
 
@@ -9078,6 +9124,7 @@ The P4 compiler should provide:
 
 * Clarified that numeric priorities cannot be assigned to entries of a table that has `const entries` (<<sec-entries>>).
 * Clarified that `switch` statements are allowed in action and function bodies, and that `switch` statements with `action_run` expressions are only allowed in control `apply` blocks (<<sec-stmts>> and <<sec-switch-stmt>>).
+* Added support for compile-time string concatenation using `++` operator (<<sec-string-type>> and <<sec-string-ops>>).
 
 === Summary of changes made in version 1.2.5, released October 11, 2024