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[binutils-gdb] gdb: Ensure compiler doesn't optimise variable out in test


https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=c67f2e1518a829c4ae4f346c316701ef5d56a30f

commit c67f2e1518a829c4ae4f346c316701ef5d56a30f
Author: Andrew Burgess <andrew.burgess@embecosm.com>
Date:   Wed Aug 29 18:49:51 2018 +0100

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

Diff:
---
 gdb/testsuite/ChangeLog           | 5 +++++
 gdb/testsuite/gdb.base/funcargs.c | 9 +++++++++
 2 files changed, 14 insertions(+)

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 0164558..23a41d4 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -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.
diff --git a/gdb/testsuite/gdb.base/funcargs.c b/gdb/testsuite/gdb.base/funcargs.c
index 600792f..d5ff192 100644
--- a/gdb/testsuite/gdb.base/funcargs.c
+++ b/gdb/testsuite/gdb.base/funcargs.c
@@ -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 ()


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