[testsuite][arm] Fix cmse-15.c expected output

[testsuite][arm] Fix cmse-15.c expected output

[Christophe Lyon]

[-- Attachment #1: Type: text/plain, Size: 1029 bytes --]

Hi,

While checking Martin's fix for PR ipa/94445, he made me realize that
the cmse-15.c testcase still fails at -Os because ICF means that we
generate
nonsecure2:
        b       nonsecure0

which is OK, but does not match the currently expected
nonsecure2:
...
        bl      __gnu_cmse_nonsecure_call

(see https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543190.html)

The test has already different expectations for v8-M and v8.1-M.

I've decided to try to use check-function-bodies to account for the
different possibilities:
- v8-M vs v8.1-M via two different prefixes
- code generation variants (-0?) via multiple regexps

I've tested that the test now passes with --target-board=-march=armv8-m.main
and --target-board=-march=armv8.1-m.main.

I feel this a bit too much of a burden for the purpose, maybe there's
a better way of handling all these alternatives (in particular,
there's a lot of duplication since the expected code for the secure*
functions is the same for v8-M and v8.1-M).

OK?

Thanks,

Christophe

[-- Attachment #2: cmse-15-function-bodies.patch.txt --]
[-- Type: text/plain, Size: 4401 bytes --]

diff --git a/gcc/testsuite/gcc.target/arm/cmse/cmse-15.c b/gcc/testsuite/gcc.target/arm/cmse/cmse-15.c
index 0e37b50..603c456 100644
--- a/gcc/testsuite/gcc.target/arm/cmse/cmse-15.c
+++ b/gcc/testsuite/gcc.target/arm/cmse/cmse-15.c
@@ -1,5 +1,9 @@
 /* { dg-do compile } */
 /* { dg-options "-mcmse" } */
+/* ARMv8-M expectation.  */
+/* { dg-final { check-function-bodies "*Noclear" "" "" { target { ! arm_cmse_clear_ok } } } } */
+/* ARMv8.1-M expectation.  */
+/* { dg-final { check-function-bodies "*Clear" "" "" { target arm_cmse_clear_ok } } } */
 
 int __attribute__ ((cmse_nonsecure_call)) (*ns_foo) (void);
 int (*s_bar) (void);
@@ -11,67 +15,242 @@ typedef int s_bar_t (void);
 typedef int __attribute__ ((cmse_nonsecure_call)) (* ns_foo_ptr) (void);
 typedef int (*s_bar_ptr) (void);
 
+/*
+*Clear nonsecure0:
+*Clear	...
+*Clear		blxns	r[0-3]
+*Clear	...
+*Noclear nonsecure0:
+*Noclear	...
+*Noclear	bl	__gnu_cmse_nonsecure_call
+*Noclear	...
+*/
 int nonsecure0 (ns_foo_t * ns_foo_p)
 {
   return ns_foo_p ();
 }
 
+/*
+*Clear nonsecure1:
+*Clear	...
+*Clear		blxns	r[0-3]
+*Clear	...
+*Noclear nonsecure1:
+*Noclear	...
+*Noclear	bl	__gnu_cmse_nonsecure_call
+*Noclear	...
+*/
 int nonsecure1 (ns_foo_t ** ns_foo_p)
 {
   return (*ns_foo_p) ();
 }
 
+/*
+*Clear nonsecure2:
+*Clear	...
+*Clear (
+*Clear		blxns	r[0-3]
+*Clear |
+*Clear	b	nonsecure0
+*Clear )
+*Clear	...
+*Noclear nonsecure2:
+*Noclear	...
+*Noclear (
+*Noclear	bl	__gnu_cmse_nonsecure_call
+*Noclear |
+*Noclear	b	nonsecure0
+*Noclear )
+*Noclear	...
+*/
 int nonsecure2 (ns_foo_ptr ns_foo_p)
 {
   return ns_foo_p ();
 }
+
+/*
+*Clear nonsecure3:
+*Clear	...
+*Clear		blxns	r[0-3]
+*Clear	...
+*Noclear nonsecure3:
+*Noclear	...
+*Noclear	bl	__gnu_cmse_nonsecure_call
+*Noclear	...
+*/
 int nonsecure3 (ns_foo_ptr * ns_foo_p)
 {
   return (*ns_foo_p) ();
 }
 
+/*
+*Clear secure0:
+*Clear	...
+*Clear (
+*Clear	bx	r[0-3]
+*Clear |
+*Clear	blx	r[0-3]
+*Clear )
+*Clear	...
+*Noclear secure0:
+*Noclear	...
+*Noclear (
+*Noclear	bx	r[0-3]
+*Noclear |
+*Noclear	blx	r[0-3]
+*Noclear )
+*Noclear	...
+*/
 int secure0 (s_bar_t * s_bar_p)
 {
   return s_bar_p ();
 }
 
+/*
+*Clear secure1:
+*Clear	...
+*Clear (
+*Clear	bx	r[0-3]
+*Clear |
+*Clear	blx	r[0-3]
+*Clear )
+*Clear	...
+*Noclear secure1:
+*Noclear	...
+*Noclear (
+*Noclear	bx	r[0-3]
+*Noclear |
+*Noclear	blx	r[0-3]
+*Noclear )
+*Noclear	...
+*/
 int secure1 (s_bar_t ** s_bar_p)
 {
   return (*s_bar_p) ();
 }
 
+/*
+*Clear secure2:
+*Clear	...
+*Clear (
+*Clear	bx	r[0-3]
+*Clear |
+*Clear	blx	r[0-3]
+*Clear |
+*Clear	b	secure0
+*Clear )
+*Clear	...
+*Noclear secure2:
+*Noclear	...
+*Noclear (
+*Noclear	bx	r[0-3]
+*Noclear |
+*Noclear	blx	r[0-3]
+*Noclear |
+*Noclear	b	secure0
+*Noclear )
+*Noclear	...
+*/
 int secure2 (s_bar_ptr s_bar_p)
 {
   return s_bar_p ();
 }
 
+/*
+*Clear secure3:
+*Clear	...
+*Clear (
+*Clear	bx	r[0-3]
+*Clear |
+*Clear	blx	r[0-3]
+*Clear )
+*Clear	...
+*Noclear secure3:
+*Noclear	...
+*Noclear (
+*Noclear	bx	r[0-3]
+*Noclear |
+*Noclear	blx	r[0-3]
+*Noclear )
+*Noclear	...
+*/
 int secure3 (s_bar_ptr * s_bar_p)
 {
   return (*s_bar_p) ();
 }
 
+/*
+*Clear nonsecure4:
+*Clear	...
+*Clear		blxns	r[0-3]
+*Clear	...
+*Noclear nonsecure4:
+*Noclear	...
+*Noclear	bl	__gnu_cmse_nonsecure_call
+*Noclear	...
+*/
 int nonsecure4 (void)
 {
   return ns_foo ();
 }
 
+/*
+*Clear nonsecure5:
+*Clear	...
+*Clear		blxns	r[0-3]
+*Clear	...
+*Noclear nonsecure5:
+*Noclear	...
+*Noclear	bl	__gnu_cmse_nonsecure_call
+*Noclear	...
+*/
 int nonsecure5 (void)
 {
   return (*ns_foo2) ();
 }
 
+/*
+*Clear secure4:
+*Clear	...
+*Clear (
+*Clear	bx	r[0-3]
+*Clear |
+*Clear	blx	r[0-3]
+*Clear )
+*Clear	...
+*Noclear secure4:
+*Noclear	...
+*Noclear (
+*Noclear	bx	r[0-3]
+*Noclear |
+*Noclear	blx	r[0-3]
+*Noclear )
+*Noclear	...
+*/
 int secure4 (void)
 {
   return s_bar ();
 }
 
+/*
+*Clear secure5:
+*Clear	...
+*Clear (
+*Clear	bx	r[0-3]
+*Clear |
+*Clear	blx	r[0-3]
+*Clear )
+*Clear	...
+*Noclear secure5:
+*Noclear	...
+*Noclear (
+*Noclear	bx	r[0-3]
+*Noclear |
+*Noclear	blx	r[0-3]
+*Noclear )
+*Noclear	...
+*/
 int secure5 (void)
 {
   return (*s_bar2) ();
 }
-
-/* ARMv8-M expectation.  */
-/* { dg-final { scan-assembler-times "bl\\s+__gnu_cmse_nonsecure_call" 6 { target { ! arm_cmse_clear_ok } } } } */
-
-/* ARMv8.1-M expectation.  */
-/* { dg-final { scan-assembler-times "blxns" 6 { target arm_cmse_clear_ok } } } */

[-- Attachment #3: cmse-15-function-bodies.chlog.txt --]
[-- Type: text/plain, Size: 137 bytes --]

2020-04-06  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/testsuite/
	* gcc.target/arm/cmse/cmse-15.c: Use check-function-bodies.

Re: [testsuite][arm] Fix cmse-15.c expected output

[Andre Vieira (lists)]

On 06/04/2020 16:12, Christophe Lyon via Gcc-patches wrote:
> Hi,
>
> While checking Martin's fix for PR ipa/94445, he made me realize that
> the cmse-15.c testcase still fails at -Os because ICF means that we
> generate
> nonsecure2:
>          b       nonsecure0
>
> which is OK, but does not match the currently expected
> nonsecure2:
> ...
>          bl      __gnu_cmse_nonsecure_call
>
> (see https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543190.html)
>
> The test has already different expectations for v8-M and v8.1-M.
>
> I've decided to try to use check-function-bodies to account for the
> different possibilities:
> - v8-M vs v8.1-M via two different prefixes
> - code generation variants (-0?) via multiple regexps
>
> I've tested that the test now passes with --target-board=-march=armv8-m.main
> and --target-board=-march=armv8.1-m.main.
>
> I feel this a bit too much of a burden for the purpose, maybe there's
> a better way of handling all these alternatives (in particular,
> there's a lot of duplication since the expected code for the secure*
> functions is the same for v8-M and v8.1-M).
>
> OK?
>
> Thanks,
>
> Christophe
Hi Christophe,

This check-function-bodies functionality is pretty sweet, I assume the ( 
A | B ) checks for either of them?
If so that looks like a good improvement. Ideally we'd also check the 
clearing for the v8.1-M cases, but that wasn't there before either and 
they would need again splitting for -mfloat-abi=soft+softfp and 
-mfloat-abi=hard.


So yeah this LGTM but you need approval from a port/global maintainer.

Cheers,
Andre

Re: [testsuite][arm] Fix cmse-15.c expected output

[Christophe Lyon]

On Tue, 7 Apr 2020 at 12:31, Andre Vieira (lists)
<andre.simoesdiasvieira@arm.com> wrote:
>
> On 06/04/2020 16:12, Christophe Lyon via Gcc-patches wrote:
> > Hi,
> >
> > While checking Martin's fix for PR ipa/94445, he made me realize that
> > the cmse-15.c testcase still fails at -Os because ICF means that we
> > generate
> > nonsecure2:
> >          b       nonsecure0
> >
> > which is OK, but does not match the currently expected
> > nonsecure2:
> > ...
> >          bl      __gnu_cmse_nonsecure_call
> >
> > (see https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543190.html)
> >
> > The test has already different expectations for v8-M and v8.1-M.
> >
> > I've decided to try to use check-function-bodies to account for the
> > different possibilities:
> > - v8-M vs v8.1-M via two different prefixes
> > - code generation variants (-0?) via multiple regexps
> >
> > I've tested that the test now passes with --target-board=-march=armv8-m.main
> > and --target-board=-march=armv8.1-m.main.
> >
> > I feel this a bit too much of a burden for the purpose, maybe there's
> > a better way of handling all these alternatives (in particular,
> > there's a lot of duplication since the expected code for the secure*
> > functions is the same for v8-M and v8.1-M).
> >
> > OK?
> >
> > Thanks,
> >
> > Christophe
> Hi Christophe,
>
> This check-function-bodies functionality is pretty sweet, I assume the (
> A | B ) checks for either of them?

Yes.

> If so that looks like a good improvement. Ideally we'd also check the
> clearing for the v8.1-M cases, but that wasn't there before either and
> they would need again splitting for -mfloat-abi=soft+softfp and
> -mfloat-abi=hard.
>
Not sure what you mean?
The only nonsecure test with the (A|B) construct is:
+*Clear nonsecure2:
+*Clear ...
+*Clear (
+*Clear blxns r[0-3]
+*Clear |
+*Clear b nonsecure0
+*Clear )

So it does check the clearing (blxns), and 'b nonsecure0' is as valid
as the result of the test for nonsecure0.


>
> So yeah this LGTM but you need approval from a port/global maintainer.
>
Thanks

> Cheers,
> Andre

Re: [testsuite][arm] Fix cmse-15.c expected output

[Richard Sandiford]

Christophe Lyon via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> Hi,
>
> While checking Martin's fix for PR ipa/94445, he made me realize that
> the cmse-15.c testcase still fails at -Os because ICF means that we
> generate
> nonsecure2:
>         b       nonsecure0
>
> which is OK, but does not match the currently expected
> nonsecure2:
> ...
>         bl      __gnu_cmse_nonsecure_call
>
> (see https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543190.html)
>
> The test has already different expectations for v8-M and v8.1-M.
>
> I've decided to try to use check-function-bodies to account for the
> different possibilities:
> - v8-M vs v8.1-M via two different prefixes
> - code generation variants (-0?) via multiple regexps
>
> I've tested that the test now passes with --target-board=-march=armv8-m.main
> and --target-board=-march=armv8.1-m.main.
>
> I feel this a bit too much of a burden for the purpose, maybe there's
> a better way of handling all these alternatives (in particular,
> there's a lot of duplication since the expected code for the secure*
> functions is the same for v8-M and v8.1-M).

FWIW, an alternative is to give multiple versions with the same prefix
and use { target ... } to select between them.  E.g.:

/*
** foo:   { target a }
**	...
*/
/*
** foo:   { target { ! a } }
**	...
*/

Thanks,
Richard

Re: [testsuite][arm] Fix cmse-15.c expected output

[Christophe Lyon]

[-- Attachment #1: Type: text/plain, Size: 2051 bytes --]

On Wed, 8 Apr 2020 at 11:48, Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Christophe Lyon via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > Hi,
> >
> > While checking Martin's fix for PR ipa/94445, he made me realize that
> > the cmse-15.c testcase still fails at -Os because ICF means that we
> > generate
> > nonsecure2:
> >         b       nonsecure0
> >
> > which is OK, but does not match the currently expected
> > nonsecure2:
> > ...
> >         bl      __gnu_cmse_nonsecure_call
> >
> > (see https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543190.html)
> >
> > The test has already different expectations for v8-M and v8.1-M.
> >
> > I've decided to try to use check-function-bodies to account for the
> > different possibilities:
> > - v8-M vs v8.1-M via two different prefixes
> > - code generation variants (-0?) via multiple regexps
> >
> > I've tested that the test now passes with --target-board=-march=armv8-m.main
> > and --target-board=-march=armv8.1-m.main.
> >
> > I feel this a bit too much of a burden for the purpose, maybe there's
> > a better way of handling all these alternatives (in particular,
> > there's a lot of duplication since the expected code for the secure*
> > functions is the same for v8-M and v8.1-M).
>
> FWIW, an alternative is to give multiple versions with the same prefix
> and use { target ... } to select between them.  E.g.:
>
> /*
> ** foo:   { target a }
> **      ...
> */
> /*
> ** foo:   { target { ! a } }
> **      ...
> */
>

Ha indeed, that makes it simpler. Thanks for the example, I hadn't
fully understand how to use that scheme: I tried to add a third
alternative (different prefix) with no selector for cases where no
distinction was needed, but I realized that all alternatives need
their full matching description.

However, {target { ! a } } does not work as is, so the attached patch
uses a non-greedy regexp to avoid trying "! a }" instead of "target {
! a }"

If OK, maybe I should commit that as two separate patches?

Thanks

Christophe


> Thanks,
> Richard

[-- Attachment #2: cmse-15-function-bodies-v2.patch.txt --]
[-- Type: text/plain, Size: 4096 bytes --]

diff --git a/gcc/testsuite/gcc.target/arm/cmse/cmse-15.c b/gcc/testsuite/gcc.target/arm/cmse/cmse-15.c
index 0e37b50..b0fefe5 100644
--- a/gcc/testsuite/gcc.target/arm/cmse/cmse-15.c
+++ b/gcc/testsuite/gcc.target/arm/cmse/cmse-15.c
@@ -1,5 +1,8 @@
 /* { dg-do compile } */
 /* { dg-options "-mcmse" } */
+/* ARMv8-M expectation with target { ! arm_cmse_clear_ok }.  */
+/* ARMv8.1-M expectation with target arm_cmse_clear_ok.  */
+/* { dg-final { check-function-bodies "**" "" "" } } */
 
 int __attribute__ ((cmse_nonsecure_call)) (*ns_foo) (void);
 int (*s_bar) (void);
@@ -11,67 +14,204 @@ typedef int s_bar_t (void);
 typedef int __attribute__ ((cmse_nonsecure_call)) (* ns_foo_ptr) (void);
 typedef int (*s_bar_ptr) (void);
 
+/*
+** nonsecure0:  { target arm_cmse_clear_ok }
+**	...
+**	blxns	r[0-3]
+**	...
+*/
+/*
+** nonsecure0: { target { ! arm_cmse_clear_ok } }
+**	...
+**	bl	__gnu_cmse_nonsecure_call
+**	...
+*/
 int nonsecure0 (ns_foo_t * ns_foo_p)
 {
   return ns_foo_p ();
 }
 
+/*
+** nonsecure1:  { target arm_cmse_clear_ok }
+**	...
+**	blxns	r[0-3]
+**	...
+*/
+/*
+** nonsecure1: { target { ! arm_cmse_clear_ok } }
+**	...
+**	bl	__gnu_cmse_nonsecure_call
+**	...
+*/
 int nonsecure1 (ns_foo_t ** ns_foo_p)
 {
   return (*ns_foo_p) ();
 }
 
+/*
+** nonsecure2:  { target arm_cmse_clear_ok }
+**	...
+** (
+**	blxns	r[0-3]
+** |
+**	b	nonsecure0
+** )
+**	...
+*/
+/*
+** nonsecure2: { target { ! arm_cmse_clear_ok } }
+**	...
+** (
+**	bl	__gnu_cmse_nonsecure_call
+** |
+**	b	nonsecure0
+** )
+**	...
+*/
 int nonsecure2 (ns_foo_ptr ns_foo_p)
 {
   return ns_foo_p ();
 }
+
+/*
+** nonsecure3:  { target arm_cmse_clear_ok }
+**	...
+**	blxns	r[0-3]
+**	...
+*/
+/*
+** nonsecure3: { target { ! arm_cmse_clear_ok } }
+**	...
+**	bl	__gnu_cmse_nonsecure_call
+**	...
+*/
 int nonsecure3 (ns_foo_ptr * ns_foo_p)
 {
   return (*ns_foo_p) ();
 }
 
+/*
+** secure0:
+**	...
+** (
+**	bx	r[0-3]
+** |
+**	blx	r[0-3]
+** )
+**	...
+*/
 int secure0 (s_bar_t * s_bar_p)
 {
   return s_bar_p ();
 }
 
+/*
+** secure1:
+**	...
+** (
+**	bx	r[0-3]
+** |
+**	blx	r[0-3]
+** )
+**	...
+*/
 int secure1 (s_bar_t ** s_bar_p)
 {
   return (*s_bar_p) ();
 }
 
+/*
+** secure2:
+**	...
+** (
+**	bx	r[0-3]
+** |
+**	blx	r[0-3]
+** |
+**	b	secure0
+** )
+**	...
+*/
 int secure2 (s_bar_ptr s_bar_p)
 {
   return s_bar_p ();
 }
 
+/*
+** secure3:
+**	...
+** (
+**	bx	r[0-3]
+** |
+**	blx	r[0-3]
+** )
+**	...
+*/
 int secure3 (s_bar_ptr * s_bar_p)
 {
   return (*s_bar_p) ();
 }
 
+/*
+** nonsecure4:  { target arm_cmse_clear_ok }
+**	...
+**	blxns	r[0-3]
+**	...
+*/
+/*
+** nonsecure4: { target { ! arm_cmse_clear_ok } }
+**	...
+**	bl	__gnu_cmse_nonsecure_call
+**	...
+*/
 int nonsecure4 (void)
 {
   return ns_foo ();
 }
 
+/*
+** nonsecure5:  { target arm_cmse_clear_ok }
+**	...
+**	blxns	r[0-3]
+**	...
+*/
+/*
+** nonsecure5: { target { ! arm_cmse_clear_ok } }
+**	...
+**	bl	__gnu_cmse_nonsecure_call
+**	...
+*/
 int nonsecure5 (void)
 {
   return (*ns_foo2) ();
 }
 
+/*
+** secure4:
+**	...
+** (
+**	bx	r[0-3]
+** |
+**	blx	r[0-3]
+** )
+**	...
+*/
 int secure4 (void)
 {
   return s_bar ();
 }
 
+/*
+** secure5:
+**	...
+** (
+**	bx	r[0-3]
+** |
+**	blx	r[0-3]
+** )
+**	...
+*/
 int secure5 (void)
 {
   return (*s_bar2) ();
 }
-
-/* ARMv8-M expectation.  */
-/* { dg-final { scan-assembler-times "bl\\s+__gnu_cmse_nonsecure_call" 6 { target { ! arm_cmse_clear_ok } } } } */
-
-/* ARMv8.1-M expectation.  */
-/* { dg-final { scan-assembler-times "blxns" 6 { target arm_cmse_clear_ok } } } */
diff --git a/gcc/testsuite/lib/scanasm.exp b/gcc/testsuite/lib/scanasm.exp
index f7d2773..0098c3d 100644
--- a/gcc/testsuite/lib/scanasm.exp
+++ b/gcc/testsuite/lib/scanasm.exp
@@ -679,7 +679,7 @@ proc check-function-bodies { args } {
 	if { [string equal -length $prefix_len $line $prefix] } {
 	    set line [string trim [string range $line $prefix_len end]]
 	    if { !$in_function } {
-		if { [regexp "^(.*\\S)\\s+{(.*)}\$" $line dummy \
+		if { [regexp "^(.*?\\S)\\s+{(.*)}\$" $line dummy \
 			  line selector] } {
 		    set selector [dg-process-target $selector]
 		} else {

[-- Attachment #3: cmse-15-function-bodies-v2.chglog.txt --]
[-- Type: text/plain, Size: 251 bytes --]

gcc/testsuite/ChangeLog:

2020-04-08  Christophe Lyon  <christophe.lyon@linaro.org>

	* gcc.target/arm/cmse/cmse-15.c: Use check-function-bodies.
	* lib/scanasm.exp (check-function-bodies): Use non-greedy regexp
	when extracting the target selector.

Re: [testsuite][arm] Fix cmse-15.c expected output

[Richard Sandiford]

Christophe Lyon <christophe.lyon@linaro.org> writes:
> On Wed, 8 Apr 2020 at 11:48, Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> Christophe Lyon via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> > Hi,
>> >
>> > While checking Martin's fix for PR ipa/94445, he made me realize that
>> > the cmse-15.c testcase still fails at -Os because ICF means that we
>> > generate
>> > nonsecure2:
>> >         b       nonsecure0
>> >
>> > which is OK, but does not match the currently expected
>> > nonsecure2:
>> > ...
>> >         bl      __gnu_cmse_nonsecure_call
>> >
>> > (see https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543190.html)
>> >
>> > The test has already different expectations for v8-M and v8.1-M.
>> >
>> > I've decided to try to use check-function-bodies to account for the
>> > different possibilities:
>> > - v8-M vs v8.1-M via two different prefixes
>> > - code generation variants (-0?) via multiple regexps
>> >
>> > I've tested that the test now passes with --target-board=-march=armv8-m.main
>> > and --target-board=-march=armv8.1-m.main.
>> >
>> > I feel this a bit too much of a burden for the purpose, maybe there's
>> > a better way of handling all these alternatives (in particular,
>> > there's a lot of duplication since the expected code for the secure*
>> > functions is the same for v8-M and v8.1-M).
>>
>> FWIW, an alternative is to give multiple versions with the same prefix
>> and use { target ... } to select between them.  E.g.:
>>
>> /*
>> ** foo:   { target a }
>> **      ...
>> */
>> /*
>> ** foo:   { target { ! a } }
>> **      ...
>> */
>>
>
> Ha indeed, that makes it simpler. Thanks for the example, I hadn't
> fully understand how to use that scheme: I tried to add a third
> alternative (different prefix) with no selector for cases where no
> distinction was needed, but I realized that all alternatives need
> their full matching description.
>
> However, {target { ! a } } does not work as is, so the attached patch
> uses a non-greedy regexp to avoid trying "! a }" instead of "target {
> ! a }"

Oops.  This part is definitely OK, thanks.

> If OK, maybe I should commit that as two separate patches?

I don't really feel qualified to review the substance of the arm part
of the patch, but given Andre's LGTM for that in the earlier version,
a rubber-stamp OK for that too.  I agree separate commits might be
better.

Thanks,
Richard