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[RFA] Fix mutual off-by-one error
- From: Keith Seitz <keiths at redhat dot com>
- To: Java Patch List <java-patches at gcc dot gnu dot org>
- Date: Fri, 27 Apr 2007 18:17:53 -0700
- Subject: [RFA] Fix mutual off-by-one error
Hi,
While working on JVMTI/JDWP exception reporting, I noticed that we have
two off-by-one errors in both the stack tracing and breakpoint code. It
just so happens that they mutually cancel each other out, so it wasn't
obvious!
The stack tracing-related bug is that _Jv_InterpFrame::get_pc returns
the PC in the interpreter. While executing any insn, the PC always
points to the NEXT instruction, not the current.
The bug in breakpoint code deals with not incrementing the PC until
AFTER the breakpoint had executed. So while stopped at a breakpoint,
"PC" actually pointed to the current instruction not the next.
With the attached patch, both JVMTI breakpoint callbacks and the stack
agree where the interpreter is stopped.
QCC? (Questions/comments/concerns - Did I just invent a new acronym?)
Keith
ChangeLog
2007-04-27 Keith Seitz <keiths@redhat.com>
* interpret-run.cc (NEXT_INSN)[DEBUG]: Advance PC before
executing the instruction.
* include/java-interp.h (_Jv_InterpFrame::get_pc): Subtract
one insn from the frame's PC. _Jv_InterpMethod::run et al
will advance the PC before executing the instruction.
Index: interpret-run.cc
===================================================================
--- interpret-run.cc (revision 124241)
+++ interpret-run.cc (working copy)
@@ -348,15 +348,16 @@
#define NEXT_INSN \
do \
{ \
+ pc_t insn = pc++; \
if (JVMTI_REQUESTED_EVENT (SingleStep)) \
{ \
JNIEnv *env = _Jv_GetCurrentJNIEnv (); \
jmethodID method = meth->self; \
- jlocation loc = meth->insn_index (pc); \
+ jlocation loc = meth->insn_index (insn); \
_Jv_JVMTI_PostEvent (JVMTI_EVENT_SINGLE_STEP, thread, \
env, method, loc); \
} \
- goto *((pc++)->insn); \
+ goto *(insn->insn); \
} \
while (0)
Index: include/java-interp.h
===================================================================
--- include/java-interp.h (revision 124241)
+++ include/java-interp.h (working copy)
@@ -465,7 +465,7 @@
else
pc = *pc_ptr;
- return pc;
+ return pc - 1;
}
};