Patch: [tree-ssa] fix REG_DEAD notes

[Dale Johannesen <dalej at apple dot com>]

On Darwin with -mpowerpc64, the call in the following example

extern unsigned long long lseek (int, unsigned long long, int);
unsigned long long x;
int foo(int f, int z) {
   return lseek(f,x,z);
}

both uses and resets R3.  gcc is not consistent about whether a REG_DEAD
note is applied in this case; life_analysis does not apply one, but 
combine
does, under some conditions.  This leads to an ICE in the scheduler,
which deletes all the REG_DEAD notes coming from combine, then
recomputes them later using life analysis, and expects the totals to
match.  This patch changes combine to be consistent with life_analysis.
Bootstrapped and tested on Darwin.

(The ICE occurs only with tree-ssa, because on mainline the merge of
the two-word return value into one register goes through  memory, and
nobody is smart enough to use a register instead, and the bug is not
tickled.  The tree-ssa optimizations cause the merge to be done in a
register, and combine eventually realizes all it needs to do is return 
R4
and R3 is dead.  A great improvement.)

There is already a testcase, gcc.dg/macho-lo-sum.c, which demonstrates
the bug; it ICEd before and passes with my patch.  I won't be adding
another testcase.

2004-02-24  Dale Johannesen  <dalej@apple.com>

        * combine.c (distribute_notes): Do not place a REG_DEAD note
        when value is both set and used.

Attachment: combinepatch.txt
Description: Text document