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

Message ID 2201f3e6-ad3c-4baa-b683-8abe84ddb668@ventanamicro.com
State New
Headers show
Series [committed,PR,rtl-optimization/115877] Fix livein computation for ext-dce | expand

Commit Message

Jeff Law July 21, 2024, 1:37 p.m. UTC 
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.


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

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index 6d4b8858ec6..c4c38659701 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -591,8 +678,10 @@  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.  */