Writing an LLVM Pass

From: Chris Lattner Date: Fri, 6 Sep 2002 02:02:58 +0000 (+0000) Subject: * Remove notes at the top of the file X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=480e2efb2e3638cd202551ed53fb456d6179d5e0;p=oota-llvm.git * Remove notes at the top of the file * Add information about how to debug a dynamically loaded pass. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3583 91177308-0d34-0410-b5e6-96231b3b80d8 --- diff --git a/docs/WritingAnLLVMPass.html b/docs/WritingAnLLVMPass.html index 3e2f167ccbb..d1cf63cae8f 100644 --- a/docs/WritingAnLLVMPass.html +++ b/docs/WritingAnLLVMPass.html @@ -1,34 +1,6 @@ Writing an LLVM Pass - - @@ -80,6 +52,11 @@ just grow as time goes on.

The releaseMemory method

Using GDB with dynamically loaded passes +

Setting a breakpoint in your pass +
Miscellaneous Problems +

Future extensions planned

Multithreaded LLVM @@ -1025,6 +1002,92 @@ internal state. This method is called after the run* method for the class, before the next call of run* in your pass.
+ +

+Using GDB with dynamically loaded passes +

+ +For sake of discussion, I'm going to assume that you are debugging a +transformation invoked by opt, although nothing described here depends +on that.

+ + +

Setting a breakpoint in your pass

+ +First thing you do is start gdb on the opt process:

+ +

+$ gdb opt
+GNU gdb 5.0
+Copyright 2000 Free Software Foundation, Inc.
+GDB is free software, covered by the GNU General Public License, and you are
+welcome to change it and/or distribute copies of it under certain conditions.
+Type "show copying" to see the conditions.
+There is absolutely no warranty for GDB.  Type "show warranty" for details.
+This GDB was configured as "sparc-sun-solaris2.6"...
+(gdb)
+

+ +Note that opt has a lot of debugging information in it, so it takes +time to load. Be patient. Since we cannot set a breakpoint in our pass yet +(the shared object isn't loaded until runtime), we must execute the process, and +have it stop before it invokes our pass, but after it has loaded the shared +object. The most foolproof way of doing this is to set a breakpoint in +PassManager::run and then run the process with the arguments you +want:

+ +

+(gdb) break PassManager::run
+Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
+(gdb) run test.bc -load /shared/lattner/cvs/llvm/lib/Debug/[libname].so -[passoption]
+Starting program: /shared/lattner/cvs/llvm/tools/Debug/opt test.bc 
+    -load /shared/lattner/cvs/llvm/lib/Debug/[libname].so -[passoption]
+Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
+70      bool PassManager::run(Module &M) { return PM->run(M); }
+(gdb)
+

+ +Once the opt stops in the PassManager::run method you are now +free to set breakpoints in your pass so that you can trace through execution or +do other standard debugging stuff.

+ + + +

Miscellaneous Problems

+ +Once you have the basics down, there are a couple of problems that GDB has, some +with solutions, some without.

+ +

Inline functions have bogus stack information. In general, GDB does a +pretty good job getting stack traces and stepping through inline functions. +When a pass is dynamically loaded however, it somehow completely loses this +capability. The only solution I know of is to de-inline a function (move it +from the body of a class to a .cpp file).
+ +
Restarting the program breaks breakpoints. After following the information +above, you have succeeded in getting some breakpoints planted in your pass. Nex +thing you know, you restart the program (i.e., you type 'run' again), +and you start getting errors about breakpoints being unsettable. The only way I +have found to "fix" this problem is to delete the breakpoints that are +already set in your pass, run the program, and re-set the breakpoints once +execution stops in PassManager::run.
+ +

+ +Hopefully these tips will help with common case debugging situations. If you'd +like to contribute some tips of your own, just contact

Chris

+ +

@@ -1098,9 +1161,9 @@ href="#Pass">Pass, only the other way around.

-
Christopher Lattner
+
Chris Lattner
-Last modified: Thu Aug 22 14:19:43 CDT 2002 +Last modified: Thu Sep 5 15:06:01 CDT 2002