From d3e5465757c599ea64f611290b7793d3141a6b7c Mon Sep 17 00:00:00 2001 From: Jason Merrill Date: Wed, 5 Oct 2022 11:50:59 -0400 Subject: gimplify: prevent some C++ temporary elision In this testcase, we were optimizing away the temporary for f(), but C++17 and above are clear that there is a temporary, and because its destructor has visible side-effects we can't optimize it away under the as-if rule. So disable this optimization for TREE_ADDRESSABLE type. I moved the declaration of volatile_p after the call to gimple_fold_indirect_ref_rhs to minimize indentation changes; I don't see any way the value of that flag could be affected by the call. gcc/ChangeLog: * gimplify.cc (gimplify_modify_expr_rhs): Don't optimize x = *(A*)& to x = for a TREE_ADDRESSABLE type. gcc/testsuite/ChangeLog: * g++.dg/init/elide9.C: New test. --- gcc/gimplify.cc | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) (limited to 'gcc/gimplify.cc') diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc index 95e16f6..d4209ea 100644 --- a/gcc/gimplify.cc +++ b/gcc/gimplify.cc @@ -5620,7 +5620,7 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p, } break; case INDIRECT_REF: - { + if (!TREE_ADDRESSABLE (TREE_TYPE (*from_p))) /* If we have code like *(const A*)(A*)&x @@ -5629,11 +5629,13 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p, of "A"), treat the entire expression as identical to "x". This kind of code arises in C++ when an object is bound to a const reference, and if "x" is a TARGET_EXPR we want - to take advantage of the optimization below. */ - bool volatile_p = TREE_THIS_VOLATILE (*from_p); - tree t = gimple_fold_indirect_ref_rhs (TREE_OPERAND (*from_p, 0)); - if (t) + to take advantage of the optimization below. But not if + the type is TREE_ADDRESSABLE; then C++17 says that the + TARGET_EXPR needs to be a temporary. */ + if (tree t + = gimple_fold_indirect_ref_rhs (TREE_OPERAND (*from_p, 0))) { + bool volatile_p = TREE_THIS_VOLATILE (*from_p); if (TREE_THIS_VOLATILE (t) != volatile_p) { if (DECL_P (t)) @@ -5646,8 +5648,7 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p, ret = GS_OK; changed = true; } - break; - } + break; case TARGET_EXPR: { -- cgit v1.1