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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. 2020-03-26 Jakub Jelinek <jakub@redhat.com> PR c++/94272 * cp-gimplify.c (cp_genericize_r): Handle STATEMENT_LIST. * g++.dg/debug/pr94272.C: New test.
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