I'm not sure which 'subsystem maintainer' to include on this as it
affects parsers for both C and c++. I've also cc'ed people from the
discussion thread.
While that ~100 message thread on the gcc list about pr24414 didn't come
to any final conclusions about clobbering registers for basic asm, there
were a few things people agreed we could do to help users right now:
- Update the basic asm docs to describe basic asm's current (and
historical) semantics (ie clobber nothing).
- Emphasize how that might be different from users' expectations or the
behavior of other compilers.
- Warn that this could change in future versions of gcc. To avoid
impacts from this change, use extended asm.
- Implement and document -Wonly-top-basic-asm (disabled by default) as a
way to locate affected statements.
This patch does these things. You can review the new doc text at
http://www.LimeGreenSocks.com/gcc/Basic-Asm.html
ChangeLog:
2016-01-24 David Wohlferd <d...@limegreensocks.com>
* doc/extend.texi: Doc basic asm behavior and new
-Wonly-top-basic-asm option.
* doc/invoke.texi: Doc new -Wonly-top-basic-asm option.
* c-family/c.opt: Define -Wonly-top-basic-asm.
* c/c-parser.c: Implement -Wonly-top-basic-asm for C.
* cp/parser.c: Implement -Wonly-top-basic-asm for c++.
* testsuite/c-c++-common/Wonly-top-basic-asm.c: New tests for
-Wonly-top-basic-asm.
* testsuite/c-c++-common/Wonly-top-basic-asm-2.c: Ditto.
Note that while I have a release on file with FSF, I don't have write
access to SVN.
dw
Index: gcc/c-family/c.opt
===================================================================
--- gcc/c-family/c.opt (revision 232773)
+++ gcc/c-family/c.opt (working copy)
@@ -585,6 +585,10 @@
C++ ObjC++ Var(warn_namespaces) Warning
Warn on namespace definition.
+Wonly-top-basic-asm
+C ObjC ObjC++ C++ Var(warn_only_top_basic_asm) Warning
+Warn on unsafe uses of basic asm.
+
Wsized-deallocation
C++ ObjC++ Var(warn_sized_deallocation) Warning EnabledBy(Wextra)
Warn about missing sized deallocation functions.
Index: gcc/c/c-parser.c
===================================================================
--- gcc/c/c-parser.c (revision 232773)
+++ gcc/c/c-parser.c (working copy)
@@ -5973,7 +5973,18 @@
labels = NULL_TREE;
if (c_parser_next_token_is (parser, CPP_CLOSE_PAREN) && !is_goto)
+ {
+ /* Warn on basic asm used inside of functions,
+ EXCEPT when in naked functions. Also allow asm(""). */
+ if (warn_only_top_basic_asm && (TREE_STRING_LENGTH (str) != 1) )
+ if (lookup_attribute ("naked",
+ DECL_ATTRIBUTES (current_function_decl))
+ == NULL_TREE)
+ warning_at(asm_loc, OPT_Wonly_top_basic_asm,
+ "asm statement in function does not use extended syntax");
+
goto done_asm;
+ }
/* Parse each colon-delimited section of operands. */
nsections = 3 + is_goto;
Index: gcc/cp/parser.c
===================================================================
--- gcc/cp/parser.c (revision 232773)
+++ gcc/cp/parser.c (working copy)
@@ -18003,6 +18003,8 @@
bool goto_p = false;
required_token missing = RT_NONE;
+ location_t asm_loc = cp_lexer_peek_token (parser->lexer)->location;
+
/* Look for the `asm' keyword. */
cp_parser_require_keyword (parser, RID_ASM, RT_ASM);
@@ -18161,6 +18163,16 @@
/* If the extended syntax was not used, mark the ASM_EXPR. */
if (!extended_p)
{
+ /* Warn on basic asm used inside of functions,
+ EXCEPT when in naked functions. Also allow asm(""). */
+ if (warn_only_top_basic_asm &&
+ (TREE_STRING_LENGTH (string) != 1))
+ if (lookup_attribute("naked",
+ DECL_ATTRIBUTES (current_function_decl))
+ == NULL_TREE)
+ warning_at(asm_loc, OPT_Wonly_top_basic_asm,
+ "asm statement in function does not use extended syntax");
+
tree temp = asm_stmt;
if (TREE_CODE (temp) == CLEANUP_POINT_EXPR)
temp = TREE_OPERAND (temp, 0);
Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi (revision 232773)
+++ gcc/doc/extend.texi (working copy)
@@ -2903,12 +2903,14 @@
although the function call is live. To keep such calls from being
optimized away, put
@smallexample
-asm ("");
+asm ("":::);
@end smallexample
@noindent
(@pxref{Extended Asm}) in the called function, to serve as a special
side-effect.
+Older code used @code{asm("")}, but newer code should use the extended
+@code{asm} format.
@item nonnull (@var{arg-index}, @dots{})
@cindex @code{nonnull} function attribute
@@ -7458,7 +7460,8 @@
@end table
@subsubheading Remarks
-Using extended @code{asm} typically produces smaller, safer, and more
+Using extended @code{asm} (@pxref{Extended Asm}) typically produces smaller,
+safer, and more
efficient code, and in most cases it is a better solution than basic
@code{asm}. However, there are two situations where only basic @code{asm}
can be used:
@@ -7487,6 +7490,8 @@
consecutive in the output, put them in a single multi-instruction @code{asm}
statement. Note that GCC's optimizers can move @code{asm} statements
relative to other code, including across jumps.
+Using inputs and outputs with extended @code{asm} can help correctly position
+your asm.
@code{asm} statements may not perform jumps into other @code{asm} statements.
GCC does not know about these jumps, and therefore cannot take
@@ -7497,6 +7502,7 @@
assembly code when optimizing. This can lead to unexpected duplicate
symbol errors during compilation if your assembly code defines symbols or
labels.
+Extended @code{asm}'s @samp{%=} may help resolve this.
Since GCC does not parse the @var{AssemblerInstructions}, it has no
visibility of any symbols it references. This may result in GCC discarding
@@ -7516,11 +7522,59 @@
Basic @code{asm} provides no
mechanism to provide different assembler strings for different dialects.
-Here is an example of basic @code{asm} for i386:
+Basic @code{asm} statements within functions do not perform an implicit
+"memory" clobber (@pxref{Clobbers}). Also, there is no implicit clobbering
+of @emph{any} registers, so (other than "naked" functions which follow the
+ABI rules) changed registers must be restored to their original value before
+exiting the @code{asm}. While this behavior has not always been documented,
+GCC has worked this way since at least v2.95.3. Also, lacking inputs and
+outputs means that GCC's optimizers may have difficulties consistently
+positioning the basic @code{asm} in the generated code.
+The concept of ``clobbering'' does not apply to basic @code{asm} statements
+outside of functions (aka top-level asm).
+
+@strong{Warning!} This "clobber nothing" behavior may be different than how
+other compilers treat basic @code{asm}, since the C standards for the
+@code{asm} statement provide no guidance regarding these semantics. As a
+result, @code{asm} statements that work correctly on other compilers may not
+work correctly with GCC (and vice versa), even though they both compile
+without error. Also, there is discussion underway about changing GCC to
+have basic @code{asm} clobber at least memory and perhaps some (or all)
+registers. If implemented, this change may fix subtle problems with
+existing @code{asm} statements. However it may break or slow down ones that
+were working correctly.
+
+If your existing code needs clobbers that GCC's basic @code{asm} is not
+providing, or if you want to 'future-proof' your asm against possible
+changes to basic @code{asm}'s semantics, use extended @code{asm}.
+Extended @code{asm} allows you to specify what (if anything) needs to be
+clobbered for your code to work correctly. You can use @ref{Warning
+Options, @option{-Wonly-top-basic-asm}} to locate basic @code{asm}
+statements that may need changes, and refer to
+@uref{https://gcc.gnu.org/wiki/ConvertBasicAsmToExtended, How to convert
+from basic asm to extended asm} for information about how to perform the
+conversion.
+
+Here is an example of top-level basic @code{asm} for i386 that defines an
+asm macro. That macro is then invoked from within a function using
+extended @code{asm}:
+
@example
-/* Note that this code will not compile with -masm=intel */
-#define DebugBreak() asm("int $3")
+/* Define macro at file scope with basic asm. */
+/* Add macro parameter p to eax. */
+asm(".macro test p\n\t"
+ "addl $\\p, %eax\n\t"
+ ".endm");
+
+/* Use macro in function using extended asm. It needs */
+/* the "cc" clobber since the flags are changed and uses */
+/* the "a" constraint since it modifies eax. */
+int DoAdd(int value)
+@{
+ asm("test 5" : "+a" (value) : : "cc");
+ return value;
+@}
@end example
@node Extended Asm
@@ -8047,7 +8101,7 @@
for @code{d} by specifying both constraints.
@anchor{FlagOutputOperands}
-@subsection Flag Output Operands
+@subsubsection Flag Output Operands
@cindex @code{asm} flag output operands
Some targets have a special register that holds the ``flags'' for the
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi (revision 232773)
+++ gcc/doc/invoke.texi (working copy)
@@ -5693,6 +5693,21 @@
a structure that has been marked with the @code{designated_init}
attribute.
+@item -Wonly-top-basic-asm @r{(C and C++ only)}
+Warn if basic @code{asm} statements are used inside a function (i.e. not at
+top-level/file scope).
+
+When used inside of functions, basic @code{asm} can result in unexpected and
+unwanted variations in behavior between compilers due to how registers are
+handled when calling the asm (@pxref{Basic Asm}). The lack of input and
+output constraints (@pxref{Extended Asm}) can also make it difficult for
+optimizers to correctly and consistently position the output relative to
+other code.
+
+Functions that are marked with the @option{naked} attribute (@pxref{Function
+Attributes}) and @code{asm} statements with an empty instruction string are
+excluded from this check.
+
@item -Whsa
Issue a warning when HSAIL cannot be emitted for the compiled function or
OpenMP construct.
