bitintlower: Fix up handling of SSA_NAME copies in coalescing [PR119722]

The following patch is miscompiled, because during the limited
SSA name coalescing the bitintlower pass does we incorrectly don't
register a conflict.
This is on
  <bb 4> [local count: 1073741824]:
  # b_17 = PHI <b_19(3), 8(2)>
  g.4_13 = g;
  _14 = g.4_13 >> 50;
  _15 = (unsigned int) _14;
  _21 = b_17;
  _16 = (unsigned int) _21;
  s_22 = _15 + _16;
  return s_22;
basic block where in the map->bitint bitmap we track 14, 17 and 19.
The build_bitint_stmt_ssa_conflicts "hook" has special code where
it tracks uses at the final statements of mergeable operations, so
e.g. the
  _16 = (unsigned int) _21;
statement is considered to be use of b_17 because _21 is not in
map->bitmap (or large_huge.m_names), i.e. is mergeable.
The problem is that build_ssa_conflict_graph has special code to handle
SSA_NAME copies and _21 = b_17; is gimple_assign_copy_p.  In such cases
it calls live_track_clear_var on the rhs1.  The problem is that
on the above bb, after we note in the _16 = (unsigned int) _21;
stmt we need b_17 the generic code makes us forget that because
of the copy statement, and then build_bitint_stmt_ssa_conflicts
ignores it completely (because _21 is large/huge bitint and is
not in map->bitint, so assumed to be handled by a later stmt in the
bb, for backwards walk like this before this one).
As the b_17 use is ignored, the coalescing thinks it can put
all of b_17, b_19 and _14 into the same partition, which is wrong,
while we can and should coalesce b_17 and b_19, _14 needs to be a different
temporary because b_17 is set before and used after _14 has been written.

The following patch fixes it by handling gimple_assign_copy_p in two
separate spots, move the generic coalesce handling of it after
build_ssa_conflict_graph (where build_ssa_conflict_graph handling
doesn't fall through to that, it does continue after the call) and
inside of build_ssa_conflict_graph it performs it too, but only if
the lhs is not mergeable large/huge bitint.

2025-04-12  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/119722
	* gimple-lower-bitint.h (build_bitint_stmt_ssa_conflicts): Add
	CLEAR argument.
	* gimple-lower-bitint.cc (build_bitint_stmt_ssa_conflicts): Add
	CLEAR argument.  Call clear on gimple_assign_copy_p rhs1 if lhs
	is large/huge bitint unless lhs is not in names.
	* tree-ssa-coalesce.cc (build_ssa_conflict_graph): Adjust
	build_bitint_stmt_ssa_conflicts caller.  Move gimple_assign_copy_p
	handling to after the build_bitint_stmt_ssa_conflicts call.

	* gcc.dg/torture/bitint-77.c: New test.

4 files changed, 60 insertions, 11 deletions

diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index 3425816..c52a657 100644
--- a/gcc/gimple-lower-bitint.cc
+++ b/gcc/gimple-lower-bitint.cc
@@ -5919,7 +5919,8 @@ build_bitint_stmt_ssa_conflicts (gimple *stmt, live_track *live,
 				 ssa_conflicts *graph, bitmap names,
 				 void (*def) (live_track *, tree,
 					      ssa_conflicts *),
-				 void (*use) (live_track *, tree))
+				 void (*use) (live_track *, tree),
+				 void (*clear) (live_track *, tree))
 {
   bool muldiv_p = false;
   tree lhs = NULL_TREE;
@@ -5936,6 +5937,25 @@ build_bitint_stmt_ssa_conflicts (gimple *stmt, live_track *live,
 	    {
 	      if (!bitmap_bit_p (names, SSA_NAME_VERSION (lhs)))
 		return;
+
+	      /* A copy between 2 partitions does not introduce an interference
+		 by itself.  If they did, you would never be able to coalesce
+		 two things which are copied.  If the two variables really do
+		 conflict, they will conflict elsewhere in the program.
+
+		 This is handled by simply removing the SRC of the copy from
+		 the live list, and processing the stmt normally.
+
+		 Don't do this if lhs is not in names though, in such cases
+		 it is actually used at some point later in the basic
+		 block.  */
+	      if (gimple_assign_copy_p (stmt))
+		{
+		  tree rhs1 = gimple_assign_rhs1 (stmt);
+		  if (TREE_CODE (rhs1) == SSA_NAME)
+		    clear (live, rhs1);
+		}
+
 	      switch (gimple_assign_rhs_code (stmt))
 		{
 		case MULT_EXPR:
diff --git a/gcc/gimple-lower-bitint.h b/gcc/gimple-lower-bitint.h
index 8662c4b..4798484 100644
--- a/gcc/gimple-lower-bitint.h
+++ b/gcc/gimple-lower-bitint.h
@@ -26,6 +26,7 @@ extern void build_bitint_stmt_ssa_conflicts (gimple *, live_track *,
 					     ssa_conflicts *, bitmap,
 					     void (*) (live_track *, tree,
 						       ssa_conflicts *),
+					     void (*) (live_track *, tree),
 					     void (*) (live_track *, tree));
 
 #endif /* GCC_GIMPLE_LOWER_BITINT_H */
diff --git a/gcc/testsuite/gcc.dg/torture/bitint-77.c b/gcc/testsuite/gcc.dg/torture/bitint-77.c
new file mode 100644
index 0000000..3e2523f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/bitint-77.c
@@ -0,0 +1,26 @@
+/* PR tree-optimization/119722 */
+/* { dg-do run { target bitint } } */
+/* { dg-options "-O2 -fno-tree-forwprop -fno-tree-copy-prop -fno-tree-fre" } */
+
+#if __BITINT_MAXWIDTH__ >= 33300
+unsigned _BitInt(33300) g;
+
+unsigned
+foo (long c)
+{
+  unsigned _BitInt(33300) b
+    = __builtin_stdc_rotate_left ((unsigned _BitInt(13)) 8, c);
+  return ((unsigned _BitInt(50)) (g >> 50)
+	  + ({ unsigned _BitInt(300) unused; b; }));
+}
+#endif
+
+int
+main ()
+{
+#if __BITINT_MAXWIDTH__ >= 33300
+  unsigned  x = foo (0);
+  if (x != 8)
+    __builtin_abort ();
+#endif
+}
diff --git a/gcc/tree-ssa-coalesce.cc b/gcc/tree-ssa-coalesce.cc
index b78ffd7..7cc39f7 100644
--- a/gcc/tree-ssa-coalesce.cc
+++ b/gcc/tree-ssa-coalesce.cc
@@ -896,6 +896,18 @@ build_ssa_conflict_graph (tree_live_info_p liveinfo)
 	  tree var;
 	  gimple *stmt = gsi_stmt (gsi);
 
+	  if (is_gimple_debug (stmt))
+	    continue;
+
+	  if (map->bitint)
+	    {
+	      build_bitint_stmt_ssa_conflicts (stmt, live, graph, map->bitint,
+					       live_track_process_def,
+					       live_track_process_use,
+					       live_track_clear_var);
+	      continue;
+	    }
+
 	  /* A copy between 2 partitions does not introduce an interference
 	     by itself.  If they did, you would never be able to coalesce
 	     two things which are copied.  If the two variables really do
@@ -912,16 +924,6 @@ build_ssa_conflict_graph (tree_live_info_p liveinfo)
                   && TREE_CODE (rhs1) == SSA_NAME)
 		live_track_clear_var (live, rhs1);
 	    }
-	  else if (is_gimple_debug (stmt))
-	    continue;
-
-	  if (map->bitint)
-	    {
-	      build_bitint_stmt_ssa_conflicts (stmt, live, graph, map->bitint,
-					       live_track_process_def,
-					       live_track_process_use);
-	      continue;
-	    }
 
 	  /* For stmts with more than one SSA_NAME definition pretend all the
 	     SSA_NAME outputs but the first one are live at this point, so