[sdcc-devel] Does stackauto make binary different everytime?

[陳韋任 <che...@g2...>]

Hi All,

  I once posted the question on sdcc-user [1], but I think I need more
technical support here. ;-)

Let me explain what I have observed first. When I enable #pragma
stackauto, I see the local
variables' stack offset different from time to time. For example, if
function foobar has the following
local variables:

    a, b, c, and d

Then the stack offset of them might be:

   sp + 2, sp + 4, sp + 6, sp + 8

or

   sp + 2, sp + 6, sp + 4, sp + 8

Although the difference doesn't affect the correctness, it does change
the binary checksum
(we use checksum to do verification). The problem comes from
redoStackOffsets [2], or more
specifically, the "BTREE_STACK" macro. After reading the code, I think
the root cause might
be the following,

  1. When BTREE_STACK is off, a linked list of stack variables is
traversed and assigned the
      stack offset. The allocation order is same each time.

  2. When BTREE_STACK is on, the allocation is done in
btree_alloc_subtree [3], and a "set"
      of stack variables is traversed and assigned the stack offset.
The order become inconsistent
      here. The set of pointers might be the culprit.

Could I just change the VertexProperties of btree from

  std::pair<std::set<symbol *>, int>

to

  std::pair<std::list<symbol *>, int>

to make the allocation order to be consistent?

  Thanks.

[1] http://comments.gmane.org/gmane.comp.compilers.sdcc.user/5145
[2] http://code.metager.de/source/xref/sdcc/sdcc/src/SDCCmem.c
[3] http://code.metager.de/source/xref/sdcc/sdcc/src/SDCCbtree.cc

-- 
Wei-Ren Chen (陳韋任)
Homepage: http://people.cs.nctu.edu.tw/~chenwj