[PR rtl-optimization/115877] Fix livein computation for ext-dce

So I'm not yet sure how I'm going to break everything down, but this is easy
enough to break out as 1/N of ext-dce fixes/improvements.

When handling uses in an insn, we first determine what bits are set in the
destination which is represented in DST_MASK.  Then we use that to refine what
bits are live in the source operands.

In the source operand handling section we *modify* DST_MASK if the source
operand is a SUBREG (ugh!).  So if the first operand is a SUBREG, then we can
incorrectly compute which bit groups are live in the second operand, especially
if it is a SUBREG as well.

This was seen when testing a larger set of patches on the rl78 port
(builtin-arith-overflow-p-7 & pr71631 execution failures), so no new test for
this bugfix.

Run through my tester (in conjunction with other ext-dce changes) on the
various cross targets.  Run individually through a bootstrap and regression
test cycle on x86_64 as well.

Pushing to the trunk.

	PR rtl-optimization/115877
gcc/
	* ext-dce.cc (ext_dce_process_uses): Restore the value of DST_MASK
	for reach operand.

1 files changed, 2 insertions, 0 deletions

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index 6d4b885..b4450e4 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -591,8 +591,10 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj, bitmap live_tmp)
 		 making things live.  Breaking from this loop will cause
 		 the iterator to work on sub-rtxs, so it is safe to break
 		 if we see something we don't know how to handle.  */
+	      unsigned HOST_WIDE_INT save_mask = dst_mask;
 	      for (;;)
 		{
+		  dst_mask = save_mask;
 		  /* Strip an outer paradoxical subreg.  The bits outside
 		     the inner mode are don't cares.  So we can just strip
 		     and process the inner object.  */