Hi! The following testcase FAILs with -fcompare-debug. The problem is that the C++ FE initially uses IF_STMTs, tcc_statement which default to TREE_SIDE_EFFECTS set, but later on is genericized into COND_EXPRs, tcc_expression which default to TREE_SIDE_EFFECTS ored from all 3 operands. Furthermore, with -g we emit by default DEBUG_BEGIN_STMTs (TREE_SIDE_EFFECTS clear) and so end up with a STATEMENT_LIST containing DEBUG_BEGIN_STMT + e.g. the IF_STMT, while with -g0 we would end up with just the IF_STMT alone and in that case there is no STATEMENT_LIST wrapping it.
Now, the STATEMENT_LIST has TREE_SIDE_EFFECTS set to match the IF_STMT, but if none of the 3 operands (condition and both branches) have TREE_SIDE_EFFECTS, genericize_if_stmt will replace the IF_STMT with COND_EXPR without TREE_SIDE_EFFECTS, but with -g only STATEMENT_LIST wrapping it will keep TREE_SIDE_EFFECTS. Then during gimplification, shortcut_cond_expr checks TREE_SIDE_EFFECTS of the operands and as it is differennt between -g and -g0, will generate different code. The following patch attempts to fix this by clearing TREE_SIDE_EFFECTS on STATEMENT_LISTs that initially have it set and contain only DEBUG_BEGIN_STMT or at most one other statement that lost TREE_SIDE_EFFECTS during the genericization. Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? 2020-03-24 Jakub Jelinek <ja...@redhat.com> PR c++/94272 * cp-gimplify.c (cp_genericize_r): Handle STATEMENT_LIST. * g++.dg/debug/pr94272.C: New test. --- gcc/cp/cp-gimplify.c.jj 2020-03-20 09:11:36.276902935 +0100 +++ gcc/cp/cp-gimplify.c 2020-03-23 14:50:08.354821919 +0100 @@ -1754,6 +1754,36 @@ cp_genericize_r (tree *stmt_p, int *walk walk_subtrees = 0; break; + case STATEMENT_LIST: + if (TREE_SIDE_EFFECTS (stmt)) + { + tree_stmt_iterator i; + int nondebug_stmts = 0; + bool clear_side_effects = true; + /* Genericization can clear TREE_SIDE_EFFECTS, e.g. when + transforming an IF_STMT into COND_EXPR. If such stmt + appears in a STATEMENT_LIST that contains only that + stmt and some DEBUG_BEGIN_STMTs, without -g where the + STATEMENT_LIST wouldn't be present at all the resulting + expression wouldn't have TREE_SIDE_EFFECTS set, so make sure + to clear it even on the STATEMENT_LIST in such cases. */ + for (i = tsi_start (stmt); !tsi_end_p (i); tsi_next (&i)) + { + tree t = tsi_stmt (i); + if (TREE_CODE (t) != DEBUG_BEGIN_STMT && nondebug_stmts < 2) + nondebug_stmts++; + cp_walk_tree (tsi_stmt_ptr (i), cp_genericize_r, data, NULL); + if (TREE_CODE (t) != DEBUG_BEGIN_STMT + && nondebug_stmts == 1 + && TREE_SIDE_EFFECTS (tsi_stmt (i))) + clear_side_effects = false; + } + if (nondebug_stmts < 2 && clear_side_effects) + TREE_SIDE_EFFECTS (stmt) = 0; + *walk_subtrees = 0; + } + break; + default: if (IS_TYPE_OR_DECL_P (stmt)) *walk_subtrees = 0; --- gcc/testsuite/g++.dg/debug/pr94272.C.jj 2020-03-23 15:04:49.037693587 +0100 +++ gcc/testsuite/g++.dg/debug/pr94272.C 2020-03-23 15:04:04.109363546 +0100 @@ -0,0 +1,14 @@ +// PR c++/94272 +// { dg-do compile } +// { dg-options "-O2 -fnon-call-exceptions -fcompare-debug" } + +int *c, d, *e; + +void +foo () +{ + if (c && d) + ; + else if (*e) + ; +} Jakub