[gcc r15-1584] PR tree-optimization/113673: Avoid load merging when potentially trapping.

[Roger Sayle]

https://gcc.gnu.org/g:d8b05aef77443e1d3d8f3f5d2c56ac49a503fee3

commit r15-1584-gd8b05aef77443e1d3d8f3f5d2c56ac49a503fee3
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Mon Jun 24 15:34:03 2024 +0100

    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:
---
 gcc/gimple-ssa-store-merging.cc |  6 ++++--
 gcc/testsuite/g++.dg/pr113673.C | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/gcc/gimple-ssa-store-merging.cc b/gcc/gimple-ssa-store-merging.cc
index cb0cb5f42f6..7dba4a7a781 100644
--- 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
index 00000000000..11489777f5b
--- /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++;
+    }
+}