[testsuite] Fix false FAIL in stap-probe.exp

  Hi,

gcc-6.2.1-1.fc26.x86_64

    # Set a breakpoint with multiple probe locations.
    gdb_test "break -pstap test:two" \
        "Breakpoint \[0-9\]+ at $hex.*2 locations.*" \
        "set multi-location probe breakpoint (probe two)"
break -pstap test:two^M
Breakpoint 2 at 0x4004e0^M
(gdb) FAIL: gdb.base/stap-probe.exp: without semaphore, optimized: set multi-location probe breakpoint (probe two)

    # Set a breakpoint with multiple probe locations.
    # In this scenario, we may expect more than 2 locations because of
    # the optimizations (inlining, loop unrolling, etc).
    gdb_test "break -pstap test:two" \
        "Breakpoint .* at $hex.*\[0-9\]+ locations.*" \
        "set multi-location probe breakpoint (probe two)"
break -pstap test:two^M
Breakpoint 2 at 0x4004e0^M
(gdb) FAIL: gdb.base/stap-probe.exp: with semaphore, optimized: set multi-location probe breakpoint (probe two)

OK for check-in?


Jan
gdb/testsuite/ChangeLog
2016-09-11  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.base/stap-probe.exp (stap_test_no_debuginfo): Try to use
	-fno-ipa-icf.
  

On Sunday, September 11 2016, Jan Kratochvil wrote:

> Hi,
>
> gcc-6.2.1-1.fc26.x86_64
>
>     # Set a breakpoint with multiple probe locations.
>     gdb_test "break -pstap test:two" \
>         "Breakpoint \[0-9\]+ at $hex.*2 locations.*" \
>         "set multi-location probe breakpoint (probe two)"
> break -pstap test:two^M
> Breakpoint 2 at 0x4004e0^M
> (gdb) FAIL: gdb.base/stap-probe.exp: without semaphore, optimized: set multi-location probe breakpoint (probe two)
>
>     # Set a breakpoint with multiple probe locations.
>     # In this scenario, we may expect more than 2 locations because of
>     # the optimizations (inlining, loop unrolling, etc).
>     gdb_test "break -pstap test:two" \
>         "Breakpoint .* at $hex.*\[0-9\]+ locations.*" \
>         "set multi-location probe breakpoint (probe two)"
> break -pstap test:two^M
> Breakpoint 2 at 0x4004e0^M
> (gdb) FAIL: gdb.base/stap-probe.exp: with semaphore, optimized: set multi-location probe breakpoint (probe two)
>
> OK for check-in?

Thanks for the patch.  While it does fix the problem, I'd prefer a
"compiler-agnostic" patch.  The problem is that m1 and m2 (the two
functions where probe 'two' is being defined) are exactly the same, so
they get optimized by GCC's ICF pass.  Since the contents of each
function are irrelevant (as long as there's a probe 'two' inside them),
I think declaring a dummy variable in one of the functions (probably
volatile) would solve the issue.  WDYT?

> Jan
>
> gdb/testsuite/ChangeLog
> 2016-09-11  Jan Kratochvil  <jan.kratochvil@redhat.com>
>
> 	* gdb.base/stap-probe.exp (stap_test_no_debuginfo): Try to use
> 	-fno-ipa-icf.
>
> diff --git a/gdb/testsuite/gdb.base/stap-probe.exp b/gdb/testsuite/gdb.base/stap-probe.exp
> index df46e80..9258926 100644
> --- a/gdb/testsuite/gdb.base/stap-probe.exp
> +++ b/gdb/testsuite/gdb.base/stap-probe.exp
> @@ -97,8 +97,11 @@ proc stap_test_no_debuginfo {exec_name {arg ""}} {
>      global testfile hex
>  
>      if {[prepare_for_testing ${testfile}.exp ${exec_name} ${testfile}.c \
> -	   {$arg nodebug optimize=-O2}]} {
> -	return -1
> +	   {$arg nodebug optimize=-O2 "additional_flags=-fno-ipa-icf"}]} {
> +	if {[prepare_for_testing ${testfile}.exp ${exec_name} ${testfile}.c \
> +	       {$arg nodebug optimize=-O2}]} {
> +	    return -1
> +	}
>      }
>  
>      if {[runto "-pstap test:user"]} {
>
  

On Sun, 11 Sep 2016 19:43:41 +0200, Sergio Durigan Junior wrote:
> Thanks for the patch.  While it does fix the problem, I'd prefer a
> "compiler-agnostic" patch.  The problem is that m1 and m2 (the two
> functions where probe 'two' is being defined) are exactly the same, so
> they get optimized by GCC's ICF pass.  Since the contents of each
> function are irrelevant (as long as there's a probe 'two' inside them),
> I think declaring a dummy variable in one of the functions (probably
> volatile) would solve the issue.  WDYT?

The problem is there are tons of optimizations the compiler can do.  If you
change anything there the compiler can do partial inlining, tail calls etc.

An unused dummy variable gets optimized out and so the problem remains.

A dummy variable would need to have __attribute__((used)) but (1) for a reason
unknown to me it does not work anyway
	gdb.base/stap-probe.c:56:2: warning: 'used' attribute ignored [-Wattributes]
and besides that (2) __attribute__ is also not much "compiler-agnostic" IMO.

My feeling from such cases is that if you try to outsmart the compiler the
next version of compiler will outsmart you some other way again.

Sure there are many ways to workaround it, I have proposed one of them.
If you want a different one you can either submit a different patch or to be
more specific how I should code the patch.  But given you should test your
idea first I guess it is more straighforward to just submit some other your
patch if you do not like the patch of mine.


Jan