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gdb: Ensure compiler doesn't optimise variable out in test

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In the test gdb.base/funcargs.exp, there's this function:

    void recurse (SVAL a, int depth)
    {
      a.s = a.i = a.l = --depth;
      if (depth == 0)
        hitbottom ();
      else
        recurse (a, depth);
    }

The test script places a breakpoint in hitbottom, and runs the
executable which calls recurse with an initial depth of 4.

When GDB hits the breakpoint in hitbottom the testscript performs a
backtrace, and examines 'a' at each level.

The problem is that 'a' is not live after either the call to
'hitbottom' or the call to 'recurse', and as a result the test fails.

In the particular case I was looking at GCC for RISC-V 32-bit, the
variable 'a' is on the stack and GCC selects the register $ra (the
return address register) to hold the pointer to 'a'.  This is fine,
because, by the time the $ra register is needed to hold a return
address (calling hitbottom or recurse) then 'a' is dead.

In this patch I propose that a use of 'a' is added after the calls to
hitbottom and recurse, this should cause the compiler to keep 'a'
around, which should ensure GDB can find it.

gdb/testsuite/ChangeLog:

	* gdb.base/funcargs.c (use_a): New function.
	(recurse): Call use_a.
This commit is contained in:
Andrew Burgess 2018-08-29 18:49:51 +01:00
parent a0dc02a6df
commit c67f2e1518
2 changed files with 14 additions and 0 deletions
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5
gdb/testsuite/ChangeLog
View File

@ -1,3 +1,8 @@
2018-08-30 Andrew Burgess <andrew.burgess@embecosm.com>
* gdb.base/funcargs.c (use_a): New function.
(recurse): Call use_a.
2018-08-29 Keith Seitz <keiths@redhat.com>
* gdb.compile/compile-cplus-anonymous.cc: New file.

9
gdb/testsuite/gdb.base/funcargs.c
View File

@ -424,6 +424,12 @@ void hitbottom ()
{
}
void use_a (SVAL a)
{
/* Trick the compiler into thinking A is important. */
volatile SVAL dummy = a;
}
void recurse (SVAL a, int depth)
{
a.s = a.i = a.l = --depth;
@ -431,6 +437,9 @@ void recurse (SVAL a, int depth)
hitbottom ();
else
recurse (a, depth);
/* Ensure A is not discarded after the above calls. */
use_a (a);
}
void test_struct_args ()