Some programing styles use a lot of inline assembler, and it is common
to use very complex preprocessor macros to generate the assembler
strings for the asm statements. In C++ there would be a typesafe alternative
using templates and constexpr to generate the assembler strings, but
unfortunately the asm statement requires plain string literals, so this
doesn't work.
This patch modifies the C++ parser to accept strings generated by
constexpr instead of just plain strings. This requires new syntax
because e.g. asm("..." : "r" (expr)) would be ambigious with a function
call. I chose () to make it unique. For example now you can write
constexpr const char *genasm() { return "insn"; }
constexpr const char *genconstraint() { return "r"; }
asm(genasm() :: (genconstraint()) (input));
The constexpr strings are allowed for the asm template, the
constraints and the clobbers (every time current asm accepts a string)
This version allows the same constexprs as C++26 static_assert,
following Jakub's suggestion.
The drawback of this scheme is that the constexpr doesn't have
full control over the input/output/clobber lists, but that can be
usually handled with a switch statement. One could imagine
more flexible ways to handle that, for example supporting constexpr
vectors for the clobber list, or similar. But even without
that it is already useful.
Bootstrapped and full test on x86_64-linux.
gcc/c-family/ChangeLog:
* c-cppbuiltin.cc (c_cpp_builtins): Define
__GNU_CONSTEXPR_ASM__.
gcc/cp/ChangeLog:
* parser.cc (cp_parser_asm_string_expression): New function to
handle constexprs for strings.
(cp_parser_using_directive): Use
cp_parser_asm_string_expression.
(cp_parser_asm_definition): Dito.
(cp_parser_yield_expression): Dito.
(cp_parser_asm_specification_opt): Dito.
(cp_parser_asm_operand_list): Dito.
(cp_parser_asm_clobber_list): Dito.
gcc/ChangeLog:
* doc/extend.texi: Document constexpr asm.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1z/constexpr-asm-1.C: New test.
* g++.dg/cpp1z/constexpr-asm-2.C: New test.
---
gcc/c-family/c-cppbuiltin.cc | 5 +-
gcc/cp/parser.cc | 81 ++++++++++++++------
gcc/doc/extend.texi | 35 +++++++--
gcc/testsuite/g++.dg/cpp1z/constexpr-asm-1.C | 30 ++++++++
gcc/testsuite/g++.dg/cpp1z/constexpr-asm-2.C | 21 +++++
5 files changed, 142 insertions(+), 30 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-asm-1.C
create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-asm-2.C
diff --git a/gcc/c-family/c-cppbuiltin.cc b/gcc/c-family/c-cppbuiltin.cc
index d9b84a0f1b97..ff2c63d6f667 100644
--- a/gcc/c-family/c-cppbuiltin.cc
+++ b/gcc/c-family/c-cppbuiltin.cc
@@ -954,7 +954,10 @@ c_cpp_builtins (cpp_reader *pfile)
}
if (cxx_dialect >= cxx11)
- cpp_define (pfile, "__GXX_EXPERIMENTAL_CXX0X__");
+ {
+ cpp_define (pfile, "__GXX_EXPERIMENTAL_CXX0X__");
+ cpp_define (pfile, "__GNU_CONSTEXPR_ASM__");
+ }
/* Binary literals have been allowed in g++ before C++11
and were standardized for C++14. */
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 779625144db4..038ed3cf424c 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -22824,6 +22824,48 @@ cp_parser_using_directive (cp_parser* parser)
cp_parser_require (parser, CPP_SEMICOLON, RT_SEMICOLON);
}
+/* Parse a string literal or constant expression yielding a string.
+ The constant expression uses extra parens to avoid ambiguity with "x"
(expr).
+ WHAT is an identifier for error messages.
+
+ asm-string-expr:
+ string-literal
+ ( constant-expr ) */
+
+static tree
+cp_parser_asm_string_expression (cp_parser *parser, const char *what)
+{
+ location_t sloc = cp_lexer_peek_token (parser->lexer)->location;
+
+ if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_PAREN))
+ {
+ matching_parens parens;
+ parens.consume_open (parser);
+ tree string = cp_parser_constant_expression (parser);
+ cstr cstr;
+ if (string != error_mark_node)
+ string = cxx_constant_value (string, tf_error);
+ if (TREE_CODE (string) == NOP_EXPR)
+ string = TREE_OPERAND (string, 0);
+ if (TREE_CODE (string) == ADDR_EXPR
+ && TREE_CODE (TREE_OPERAND (string, 0)) == STRING_CST)
+ string = TREE_OPERAND (string, 0);
+ if (TREE_CODE (string) == VIEW_CONVERT_EXPR)
+ string = TREE_OPERAND (string, 0);
+ if (!get_cstr (string, sloc, "asm", what, cstr))
+ return error_mark_node;
+ const char *msg;
+ int len;
+ if (!extract_cstr ("asm", what, sloc, cstr, msg, len))
+ return error_mark_node;
+ parens.require_close (parser);
+ string = build_string (len, msg);
+ free_cstr (cstr);
+ return string;
+ }
+ return cp_parser_string_literal (parser, false, false);
+}
+
/* Parse an asm-definition.
asm-qualifier:
@@ -22836,19 +22878,19 @@ cp_parser_using_directive (cp_parser* parser)
asm-qualifier-list asm-qualifier
asm-definition:
- asm ( string-literal ) ;
+ asm ( constant-expr ) ;
GNU Extension:
asm-definition:
- asm asm-qualifier-list [opt] ( string-literal ) ;
- asm asm-qualifier-list [opt] ( string-literal : asm-operand-list [opt] ) ;
- asm asm-qualifier-list [opt] ( string-literal : asm-operand-list [opt]
+ asm asm-qualifier-list [opt] ( asm-string-expr ) ;
+ asm asm-qualifier-list [opt] ( asm-string-expr : asm-operand-list [opt] )
;
+ asm asm-qualifier-list [opt] ( asm-string-expr : asm-operand-list [opt]
: asm-operand-list [opt] ) ;
- asm asm-qualifier-list [opt] ( string-literal : asm-operand-list [opt]
+ asm asm-qualifier-list [opt] ( asm-string-expr : asm-operand-list [opt]
: asm-operand-list [opt]
: asm-clobber-list [opt] ) ;
- asm asm-qualifier-list [opt] ( string-literal : : asm-operand-list [opt]
+ asm asm-qualifier-list [opt] ( asm-string-expr : : asm-operand-list [opt]
: asm-clobber-list [opt]
: asm-goto-list ) ;
@@ -22967,8 +23009,7 @@ cp_parser_asm_definition (cp_parser* parser)
if (!cp_parser_require (parser, CPP_OPEN_PAREN, RT_OPEN_PAREN))
return;
/* Look for the string. */
- tree string = cp_parser_string_literal (parser, /*translate=*/false,
- /*wide_ok=*/false);
+ tree string = cp_parser_asm_string_expression (parser, "instruction");
if (string == error_mark_node)
{
cp_parser_skip_to_closing_parenthesis (parser, true, false,
@@ -29627,7 +29668,7 @@ cp_parser_yield_expression (cp_parser* parser)
/* Parse an (optional) asm-specification.
asm-specification:
- asm ( string-literal )
+ asm ( asm-string-expr )
If the asm-specification is present, returns a STRING_CST
corresponding to the string-literal. Otherwise, returns
@@ -29650,9 +29691,7 @@ cp_parser_asm_specification_opt (cp_parser* parser)
parens.require_open (parser);
/* Look for the string-literal. */
- tree asm_specification = cp_parser_string_literal (parser,
- /*translate=*/false,
- /*wide_ok=*/false);
+ tree asm_specification = cp_parser_asm_string_expression (parser,
"instruction");
/* Look for the `)'. */
parens.require_close (parser);
@@ -29667,8 +29706,8 @@ cp_parser_asm_specification_opt (cp_parser* parser)
asm-operand-list , asm-operand
asm-operand:
- string-literal ( expression )
- [ string-literal ] string-literal ( expression )
+ asm-string-expr ( expression )
+ [ asm-string-expr ] asm-string-expr ( expression )
Returns a TREE_LIST representing the operands. The TREE_VALUE of
each node is the expression. The TREE_PURPOSE is itself a
@@ -29701,10 +29740,8 @@ cp_parser_asm_operand_list (cp_parser* parser)
}
else
name = NULL_TREE;
- /* Look for the string-literal. */
- tree string_literal = cp_parser_string_literal (parser,
- /*translate=*/false,
- /*wide_ok=*/false);
+ /* Look for the string. */
+ tree string_literal = cp_parser_asm_string_expression (parser,
"operand");
/* Look for the `('. */
matching_parens parens;
@@ -29737,8 +29774,8 @@ cp_parser_asm_operand_list (cp_parser* parser)
/* Parse an asm-clobber-list.
asm-clobber-list:
- string-literal
- asm-clobber-list , string-literal
+ const-expression
+ asm-clobber-list , const-expression
Returns a TREE_LIST, indicating the clobbers in the order that they
appeared. The TREE_VALUE of each node is a STRING_CST. */
@@ -29751,9 +29788,7 @@ cp_parser_asm_clobber_list (cp_parser* parser)
while (true)
{
/* Look for the string literal. */
- tree string_literal = cp_parser_string_literal (parser,
- /*translate=*/false,
- /*wide_ok=*/false);
+ tree string_literal = cp_parser_asm_string_expression (parser,
"clobber");
/* Add it to the list. */
clobbers = tree_cons (NULL_TREE, string_literal, clobbers);
/* If the next token is not a `,', then the list is
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 799a36586dc9..023b17f600ec 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -10700,14 +10700,30 @@ contain any instructions recognized by the assembler,
including directives.
GCC does not parse the assembler instructions themselves and
does not know what they mean or even whether they are valid assembler input.
-You may place multiple assembler instructions together in a single @code{asm}
-string, separated by the characters normally used in assembly code for the
-system. A combination that works in most places is a newline to break the
+You may place multiple assembler instructions together in a single @code{asm}
+string, separated by the characters normally used in assembly code for the
+system. A combination that works in most places is a newline to break the
line, plus a tab character (written as @samp{\n\t}).
-Some assemblers allow semicolons as a line separator. However,
-note that some assembler dialects use semicolons to start a comment.
+Some assemblers allow semicolons as a line separator. However,
+note that some assembler dialects use semicolons to start a comment.
@end table
+@node asm constexprs
+With gnu++11 or later the string can also be a compile time constant expression
+inside parens. The constant expression can return a string or a container
+with data and size members, following similar rules as C++26
@code{static_assert}
+message. Any string is converted to the character set of the source code.
+When this feature is available the @code{__GNU_CONSTEXPR_ASM__} cpp symbol is
defined.
+
+@example
+constexpr const char *genfoo() @{ return "foo"; @}
+
+void function()
+@{
+ asm((genfoo()));
+@}
+@end example
+
@subsubheading Remarks
Using extended @code{asm} (@pxref{Extended Asm}) typically produces
smaller, safer, and more efficient code, and in most cases it is a
@@ -10850,20 +10866,27 @@ perform a jump to one of the labels listed in the
@var{GotoLabels}.
@item AssemblerTemplate
This is a literal string that is the template for the assembler code. It is a
combination of fixed text and tokens that refer to the input, output,
-and goto parameters. @xref{AssemblerTemplate}.
+and goto parameters. @xref{AssemblerTemplate}. With gnu++11 or later it can
+also be a constant expression inside parens (see @ref{asm constexprs}).
@item OutputOperands
A comma-separated list of the C variables modified by the instructions in the
@var{AssemblerTemplate}. An empty list is permitted. @xref{OutputOperands}.
+With gnu++11 or later the strings can also be constant expressions inside
parens
+(see @ref{asm constexprs})
@item InputOperands
A comma-separated list of C expressions read by the instructions in the
@var{AssemblerTemplate}. An empty list is permitted. @xref{InputOperands}.
+With gnu++11 or later the strings can also be constant expressions inside
parens
+(see @ref{asm constexprs})
@item Clobbers
A comma-separated list of registers or other values changed by the
@var{AssemblerTemplate}, beyond those listed as outputs.
An empty list is permitted. @xref{Clobbers and Scratch Registers}.
+With gnu++11 or later the strings can also be constant expressions inside
parens
+(see @ref{asm constexprs})
@item GotoLabels
When you are using the @code{goto} form of @code{asm}, this section contains
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-asm-1.C
b/gcc/testsuite/g++.dg/cpp1z/constexpr-asm-1.C
new file mode 100644
index 000000000000..b1305571a2c1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-asm-1.C
@@ -0,0 +1,30 @@
+/* { dg-do compile } */
+/* { dg-options "-std=gnu++11" } */
+
+constexpr const char *genfoo()
+{
+ return "foo %1,%0";
+}
+
+constexpr const char *genoutput()
+{
+ return "=r";
+}
+
+constexpr const char *geninput()
+{
+ return "r";
+}
+
+constexpr const char *genclobber()
+{
+ return "memory";
+}
+
+void f()
+{
+ int a;
+ asm((genfoo()) : (genoutput()) (a) : (geninput()) (1) : (genclobber()));
+}
+
+/* { dg-final { scan-assembler "foo" } } */
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-asm-2.C
b/gcc/testsuite/g++.dg/cpp1z/constexpr-asm-2.C
new file mode 100644
index 000000000000..a34fd14ec748
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-asm-2.C
@@ -0,0 +1,21 @@
+/* { dg-do compile } */
+/* { dg-options "-std=gnu++11" } */
+
+using size_t = typeof(sizeof(0));
+template <typename T, size_t N>
+struct array {
+ constexpr size_t size () const { return N; }
+ constexpr const T *data () const { return a; }
+ const T a[N];
+};
+
+void f()
+{
+ int a;
+ asm((array<char, 3> {'f','o','o'}) :
+ (array<char, 2>{'=','r'}) (a) :
+ (array<char, 1>{'r'}) (1) :
+ (array<char, 6>{'m','e','m','o','r','y'}));
+}
+
+/* { dg-final { scan-assembler "foo" } } */
--
2.45.1
