This middle-end patch resolves PR c/106264 which is a spurious warning regression caused by the tree-level expansion of modf, frexp and remquo producing "expression has no-effect" when the built-in function's result is ignored. When these built-ins were first expanded at tree-level, fold_builtin_n would blindly set TREE_NO_WARNING for all built-ins. Now that we're more discerning, we should precisely set TREE_NO_WARNING selectively on those COMPOUND_EXPRs that need them.
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap and make -k check with no new failures. Ok for mainline? 2022-07-16 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR c/106264 * builtins.cc (fold_builtin_frexp): Set TREE_NO_WARNING on COMPOUND_EXPR to silence spurious warning if result isn't used. (fold_builtin_modf): Likewise. (do_mpfr_remquo): Likewise. gcc/testsuite/ChangeLog PR c/106264 * gcc.dg/pr106264.c: New test case. Thanks in advance, Roger --
diff --git a/gcc/builtins.cc b/gcc/builtins.cc index 35b9197..c745777 100644 --- a/gcc/builtins.cc +++ b/gcc/builtins.cc @@ -8625,7 +8625,7 @@ fold_builtin_frexp (location_t loc, tree arg0, tree arg1, tree rettype) if (TYPE_MAIN_VARIANT (TREE_TYPE (arg1)) == integer_type_node) { const REAL_VALUE_TYPE *const value = TREE_REAL_CST_PTR (arg0); - tree frac, exp; + tree frac, exp, res; switch (value->cl) { @@ -8656,7 +8656,9 @@ fold_builtin_frexp (location_t loc, tree arg0, tree arg1, tree rettype) /* Create the COMPOUND_EXPR (*arg1 = trunc, frac). */ arg1 = fold_build2_loc (loc, MODIFY_EXPR, rettype, arg1, exp); TREE_SIDE_EFFECTS (arg1) = 1; - return fold_build2_loc (loc, COMPOUND_EXPR, rettype, arg1, frac); + res = fold_build2_loc (loc, COMPOUND_EXPR, rettype, arg1, frac); + TREE_NO_WARNING (res) = 1; + return res; } return NULL_TREE; @@ -8682,6 +8684,7 @@ fold_builtin_modf (location_t loc, tree arg0, tree arg1, tree rettype) { const REAL_VALUE_TYPE *const value = TREE_REAL_CST_PTR (arg0); REAL_VALUE_TYPE trunc, frac; + tree res; switch (value->cl) { @@ -8711,8 +8714,10 @@ fold_builtin_modf (location_t loc, tree arg0, tree arg1, tree rettype) arg1 = fold_build2_loc (loc, MODIFY_EXPR, rettype, arg1, build_real (rettype, trunc)); TREE_SIDE_EFFECTS (arg1) = 1; - return fold_build2_loc (loc, COMPOUND_EXPR, rettype, arg1, - build_real (rettype, frac)); + res = fold_build2_loc (loc, COMPOUND_EXPR, rettype, arg1, + build_real (rettype, frac)); + TREE_NO_WARNING (res) = 1; + return res; } return NULL_TREE; @@ -10673,8 +10678,10 @@ do_mpfr_remquo (tree arg0, tree arg1, tree arg_quo) integer_quo)); TREE_SIDE_EFFECTS (result_quo) = 1; /* Combine the quo assignment with the rem. */ - result = non_lvalue (fold_build2 (COMPOUND_EXPR, type, - result_quo, result_rem)); + result = fold_build2 (COMPOUND_EXPR, type, + result_quo, result_rem); + TREE_NO_WARNING (result) = 1; + result = non_lvalue (result); } } } diff --git a/gcc/testsuite/gcc.dg/pr106264.c b/gcc/testsuite/gcc.dg/pr106264.c new file mode 100644 index 0000000..6b4af49 --- /dev/null +++ b/gcc/testsuite/gcc.dg/pr106264.c @@ -0,0 +1,27 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 -Wall" } */ +double frexp (double, int*); +double modf (double, double*); +double remquo (double, double, int*); + +int f (void) +{ + int y; + frexp (1.0, &y); + return y; +} + +double g (void) +{ + double y; + modf (1.0, &y); + return y; +} + +int h (void) +{ + int y; + remquo (1.0, 1.0, &y); + return y; +} +