PR tree-optimization/113673: Avoid load merging when potentially trapping.

This patch fixes PR tree-optimization/113673, a P2 ice-on-valid regression
caused by load merging of (ptr[0]<<8)+ptr[1] when -ftrapv has been
specified.  When the operator is | or ^ this is safe, but for addition
of signed integer types, a trap may be generated/required, so merging this
idiom into a single non-trapping instruction is inappropriate, confusing
the compiler by transforming a basic block with an exception edge into one
without.

This revision implements Richard Biener's feedback to add an early check
for stmt_can_throw_internal (cfun, stmt) to prevent transforming in the
presence of any statement that could trap, not just overflow on addition.
The one other tweak included in this patch is to mark the local function
find_bswap_or_nop_load as static ensuring that it isn't called from outside
this file, and guaranteeing that it is dominated by stmt_can_throw_internal
checking.

2024-06-24  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR tree-optimization/113673
	* gimple-ssa-store-merging.cc (find_bswap_or_nop_load): Make static.
	(find_bswap_or_nop_1): Avoid transformations (load merging) when
	stmt_can_throw_internal indicates that a statement can trap.

gcc/testsuite/ChangeLog
	PR tree-optimization/113673
	* g++.dg/pr113673.C: New test case.

diff --git a/gcc/gimple-ssa-store-merging.cc b/gcc/gimple-ssa-store-merging.cc
index cb0cb5f42f65c4a2d23454c632078d6e25c59abc..7dba4a7a781f88019c177276036c0e5e3152df44 100644 (file)
--- a/gcc/gimple-ssa-store-merging.cc
+++ b/gcc/gimple-ssa-store-merging.cc
@@ -363,7 +363,7 @@ init_symbolic_number (struct symbolic_number *n, tree src)
    the answer. If so, REF is that memory source and the base of the memory area
    accessed and the offset of the access from that base are recorded in N.  */
 
-bool
+static bool
 find_bswap_or_nop_load (gimple *stmt, tree ref, struct symbolic_number *n)
 {
   /* Leaf node is an array or component ref. Memorize its base and
@@ -610,7 +610,9 @@ find_bswap_or_nop_1 (gimple *stmt, struct symbolic_number *n, int limit)
   gimple *rhs1_stmt, *rhs2_stmt, *source_stmt1;
   enum gimple_rhs_class rhs_class;
 
-  if (!limit || !is_gimple_assign (stmt))
+  if (!limit
+      || !is_gimple_assign (stmt)
+      || stmt_can_throw_internal (cfun, stmt))
     return NULL;
 
   rhs1 = gimple_assign_rhs1 (stmt);

diff --git a/gcc/testsuite/g++.dg/pr113673.C b/gcc/testsuite/g++.dg/pr113673.C
new file mode 100644 (file)
index 0000000..1148977
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr113673.C
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-Os -fnon-call-exceptions -ftrapv" } */
+
+struct s { ~s(); };
+void
+h (unsigned char *data, int c)
+{
+  s a1;
+  while (c)
+    {
+      int m = *data++ << 8;
+      m += *data++;
+    }
+}