Hi!
The following testcases emit a bogus -Wconversion warning. This is because
conversion_warning function doesn't handle BIT_*_EXPR (only unsafe_conversion_p
that is called during the default: case, and that one doesn't handle
SAVE_EXPRs added because the unsigned char & or | operands promoted to int
have side-effects and =| or =& is used.
The patch handles BIT_IOR_EXPR/BIT_XOR_EXPR like the last 2 operands of
COND_EXPR by recursing on the two operands, if either of them doesn't fit
into the narrower type, complain. BIT_AND_EXPR too, but first it needs to
handle some special cases that unsafe_conversion_p does, namely when one
of the two operands is a constant.
This fixes completely the pr101537.c test and for C also pr103881.c
and doesn't regress anything in the testsuite, for C++ pr103881.c still
emits the bogus warnings.
This is because while the C FE emits in that case a SAVE_EXPR that
conversion_warning can handle already, C++ FE emits
TARGET_EXPR <D.whatever, ...>, something | D.whatever
etc. and conversion_warning handles COMPOUND_EXPR by "recursing" on the
rhs. To handle that case, we'd need for TARGET_EXPR on the lhs remember
in some hash map the mapping from D.whatever to the TARGET_EXPR and when
we see D.whatever, use corresponding TARGET_EXPR initializer instead.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2022-01-04 Jakub Jelinek <jakub@redhat.com>
PR c/101537
PR c/103881
gcc/c-family/
* c-warn.c (conversion_warning): Handle BIT_AND_EXPR, BIT_IOR_EXPR
and BIT_XOR_EXPR.
gcc/testsuite/
* c-c++-common/pr101537.c: New test.
* c-c++-common/pr103881.c: New test.
--- gcc/c-family/c-warn.c.jj 2022-01-03 10:40:49.745044771 +0100
+++ gcc/c-family/c-warn.c 2022-01-03 12:42:55.174012944 +0100
@@ -1304,6 +1304,34 @@ conversion_warning (location_t loc, tree
|| conversion_warning (loc, type, op2, result));
}
+ case BIT_AND_EXPR:
+ if (TREE_CODE (expr_type) == INTEGER_TYPE
+ && TREE_CODE (type) == INTEGER_TYPE)
+ for (int i = 0; i < 2; ++i)
+ {
+ tree op = TREE_OPERAND (expr, i);
+ if (TREE_CODE (op) != INTEGER_CST)
+ continue;
+
+ /* If one of the operands is a non-negative constant
+ that fits in the target type, then the type of the
+ other operand does not matter. */
+ if (int_fits_type_p (op, c_common_signed_type (type))
+ && int_fits_type_p (op, c_common_unsigned_type (type)))
+ return false;
+
+ /* If constant is unsigned and fits in the target
+ type, then the result will also fit. */
+ if (TYPE_UNSIGNED (TREE_TYPE (op)) && int_fits_type_p (op, type))
+ return false;
+ }
+ /* FALLTHRU */
+ case BIT_IOR_EXPR:
+ case BIT_XOR_EXPR:
+ return (conversion_warning (loc, type, TREE_OPERAND (expr, 0), result)
+ || conversion_warning (loc, type, TREE_OPERAND (expr, 1),
+ result));
+
default_:
default:
conversion_kind = unsafe_conversion_p (type, expr, result, true);
--- gcc/testsuite/c-c++-common/pr101537.c.jj 2022-01-03 12:08:33.781823852 +0100
+++ gcc/testsuite/c-c++-common/pr101537.c 2022-01-03 12:08:33.781823852 +0100
@@ -0,0 +1,26 @@
+/* PR c/101537 */
+/* { dg-do compile } */
+/* { dg-options "-Wconversion" } */
+
+int
+foo ()
+{
+ int aaa = 1;
+ unsigned char bbb = 0;
+ bbb |= aaa ? 1 : 0;
+ return bbb;
+}
+
+int
+bar (unsigned char x, int f)
+{
+ x |= f ? 1 : 0;
+ return x;
+}
+
+int
+baz (unsigned char x, int f)
+{
+ x = x | f ? 1 : 0;
+ return x;
+}
--- gcc/testsuite/c-c++-common/pr103881.c.jj 2022-01-03 12:08:33.781823852 +0100
+++ gcc/testsuite/c-c++-common/pr103881.c 2022-01-03 12:08:33.781823852 +0100
@@ -0,0 +1,20 @@
+/* PR c/103881 */
+/* { dg-do compile } */
+/* { dg-options "-Wconversion" } */
+
+unsigned char bar (void);
+
+void
+foo (void)
+{
+ unsigned char t = 0;
+ t |= bar ();
+ t |= bar () & bar (); /* { dg-bogus "conversion from 'int' to 'unsigned char' may change value" "" { xfail c++ } } */
+ t &= bar () & bar (); /* { dg-bogus "conversion from 'int' to 'unsigned char' may change value" "" { xfail c++ } } */
+ t = bar () & bar ();
+
+ unsigned char a = bar ();
+ t |= a & a;
+ t |= bar () & a; /* { dg-bogus "conversion from 'int' to 'unsigned char' may change value" "" { xfail c++ } } */
+ t |= a & bar (); /* { dg-bogus "conversion from 'int' to 'unsigned char' may change value" "" { xfail c++ } } */
+}
Jakub