[PATCH 1/2] gdbsupport: ignore -Wenum-constexpr-conversion in enum-flags.h

[PATCH 1/2] gdbsupport: ignore -Wenum-constexpr-conversion in enum-flags.h

[Simon Marchi]

When building with clang 16, we get:

      CXX    gdb.o
    In file included from /home/smarchi/src/binutils-gdb/gdb/gdb.c:19:
    In file included from /home/smarchi/src/binutils-gdb/gdb/defs.h:65:
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/enum-flags.h:95:52: error: integer value -1 is outside the valid range of values [0, 15] for this enumeration type [-Wenum-constexpr-conversion]
        integer_for_size<sizeof (T), static_cast<bool>(T (-1) < T (0))>::type
                                                       ^

The error message does not make it clear in the context of which enum
flag this fails (i.e. what is T in this context), but it doesn't really
matter, we have similar warning/errors for many of them, if we let the
build go through.

clang is right that the value -1 is invalid for the enum type we cast -1
to.  However, we do need this expression in order to select an integer
type with the appropriate signedness.  That is, with the same signedness
as the underlying type of the enum.

I first wondered if that was really needed, if we couldn't use
std::underlying_type for that.  It turns out that the comment just above
says:

    /* Note that std::underlying_type<enum_type> is not what we want here,
       since that returns unsigned int even when the enum decays to signed
       int.  */

I was surprised, because std::is_signed<std::underlying_type<enum_type>>
returns the right thing.  So I tried replacing all this with
std::underlying_type, see if that would work.  Doing so causes some
build failures in unittests/enum-flags-selftests.c:

      CXX    unittests/enum-flags-selftests.o
    /home/smarchi/src/binutils-gdb/gdb/unittests/enum-flags-selftests.c:254:1: error: static assertion failed due to requirement 'gdb::is_same<selftests::enum_flags_tests::check_valid_expr254::archetype<enum_flags<s
    elftests::enum_flags_tests::RE>, selftests::enum_flags_tests::RE, enum_flags<selftests::enum_flags_tests::RE2>, selftests::enum_flags_tests::RE2, enum_flags<selftests::enum_flags_tests::URE>, selftests::enum_fla
    gs_tests::URE, int>, selftests::enum_flags_tests::check_valid_expr254::archetype<enum_flags<selftests::enum_flags_tests::RE>, selftests::enum_flags_tests::RE, enum_flags<selftests::enum_flags_tests::RE2>, selfte
    sts::enum_flags_tests::RE2, enum_flags<selftests::enum_flags_tests::URE>, selftests::enum_flags_tests::URE, unsigned int>>::value == true':
    CHECK_VALID (true,  int,  true ? EF () : EF2 ())
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/unittests/enum-flags-selftests.c:91:3: note: expanded from macro 'CHECK_VALID'
      CHECK_VALID_EXPR_6 (EF, RE, EF2, RE2, UEF, URE, VALID, EXPR_TYPE, EXPR)
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/valid-expr.h:105:3: note: expanded from macro 'CHECK_VALID_EXPR_6'
      CHECK_VALID_EXPR_INT (ESC_PARENS (typename T1, typename T2,           \
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/valid-expr.h:66:3: note: expanded from macro 'CHECK_VALID_EXPR_INT'
      static_assert (gdb::is_detected_exact<archetype<TYPES, EXPR_TYPE>,    \
      ^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is a bit hard to decode, but basically enumerations have the
following funny property that they decay into a signed int, even if
their implicit underlying type is unsigned.  This code:

    enum A {};
    enum B {};

    int main() {
      std::cout << std::is_signed<std::underlying_type<A>::type>::value
                << std::endl;
      std::cout << std::is_signed<std::underlying_type<B>::type>::value
                << std::endl;
      auto result = true ? A() : B();
      std::cout << std::is_signed<decltype(result)>::value << std::endl;
    }

produces:

    0
    0
    1

So, the "CHECK_VALID" above checks that this property works for enum flags the
same way as it would if you were using their underlying enum types.  And
somehow, changing integer_for_size to use std::underlying_type breaks that.

Since the current code does what we want, and I don't see any way of doing it
differently, ignore -Wenum-constexpr-conversion around it.

Change-Id: Ibc82ae7bbdb812102ae3f1dd099fc859dc6f3cc2
---
 gdbsupport/enum-flags.h | 3 +++
 include/diagnostics.h   | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/gdbsupport/enum-flags.h b/gdbsupport/enum-flags.h
index 700037f61260..41ac7838f060 100644
--- a/gdbsupport/enum-flags.h
+++ b/gdbsupport/enum-flags.h
@@ -91,9 +91,12 @@ template<> struct integer_for_size<8, 1> { typedef int64_t type; };
 template<typename T>
 struct enum_underlying_type
 {
+  DIAGNOSTIC_PUSH
+  DIAGNOSTIC_IGNORE_ENUM_CONSTEXPR_CONVERSION
   typedef typename
     integer_for_size<sizeof (T), static_cast<bool>(T (-1) < T (0))>::type
     type;
+  DIAGNOSTIC_POP
 };
 
 namespace enum_flags_detail
diff --git a/include/diagnostics.h b/include/diagnostics.h
index d3ff27bc0080..41e6db65391f 100644
--- a/include/diagnostics.h
+++ b/include/diagnostics.h
@@ -76,6 +76,11 @@
 # define DIAGNOSTIC_ERROR_SWITCH \
   DIAGNOSTIC_ERROR ("-Wswitch")
 
+# if __has_warning ("-Wenum-constexpr-conversion")
+#  define DIAGNOSTIC_IGNORE_ENUM_CONSTEXPR_CONVERSION \
+   DIAGNOSTIC_IGNORE ("-Wenum-constexpr-conversion")
+# endif
+
 #elif defined (__GNUC__) /* GCC */
 
 # define DIAGNOSTIC_IGNORE_DEPRECATED_DECLARATIONS \
@@ -155,4 +160,8 @@
 # define DIAGNOSTIC_ERROR_SWITCH
 #endif
 
+#ifndef DIAGNOSTIC_IGNORE_ENUM_CONSTEXPR_CONVERSION
+# define DIAGNOSTIC_IGNORE_ENUM_CONSTEXPR_CONVERSION
+#endif
+
 #endif /* DIAGNOSTICS_H */
-- 
2.39.2

[PATCH 2/2] gdb: fix -Wsingle-bit-bitfield-constant-conversion warning in z80-tdep.c

[Simon Marchi]

When building with clang 16, I see:

    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:338:32: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
            info->prologue_type.load_args = 1;
                                          ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:345:36: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.critical = 1;
                                       ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:351:37: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.interrupt = 1;
                                        ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:367:36: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
                  info->prologue_type.fp_sdcc = 1;
                                              ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:375:35: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.fp_sdcc = 1;
                                      ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:380:35: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.fp_sdcc = 1;
                                      ^ ~

Fix that by using "unsigned int" as the bitfield's underlying type.

Change-Id: I3550a0112f993865dc70b18f02ab11bb5012693d
---
 gdb/z80-tdep.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/gdb/z80-tdep.c b/gdb/z80-tdep.c
index 9dc24df5ed1c..5c66331efcdf 100644
--- a/gdb/z80-tdep.c
+++ b/gdb/z80-tdep.c
@@ -98,11 +98,11 @@ struct z80_unwind_cache
 
   struct
   {
-    int called:1;	/* there is return address on stack */
-    int load_args:1;	/* prologues loads args using POPs */
-    int fp_sdcc:1;	/* prologue saves and adjusts frame pointer IX */
-    int interrupt:1;	/* __interrupt handler */
-    int critical:1;	/* __critical function */
+    unsigned int called : 1;    /* there is return address on stack */
+    unsigned int load_args : 1; /* prologues loads args using POPs */
+    unsigned int fp_sdcc : 1;   /* prologue saves and adjusts frame pointer IX */
+    unsigned int interrupt : 1; /* __interrupt handler */
+    unsigned int critical : 1;  /* __critical function */
   } prologue_type;
 
   /* Table indicating the location of each and every register.  */
-- 
2.39.2

Re: [PATCH 1/2] gdbsupport: ignore -Wenum-constexpr-conversion in enum-flags.h

[Simon Marchi]

On 2/23/23 12:35, Simon Marchi via Gdb-patches wrote:
> When building with clang 16, we get:

I pushed these 2 patches.

Simon

[PATCH 2/2] gdb: fix -Wsingle-bit-bitfield-constant-conversion warning in z80-tdep.c

[Simon Marchi]

When building with clang 16, I see:

    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:338:32: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
            info->prologue_type.load_args = 1;
                                          ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:345:36: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.critical = 1;
                                       ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:351:37: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.interrupt = 1;
                                        ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:367:36: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
                  info->prologue_type.fp_sdcc = 1;
                                              ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:375:35: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.fp_sdcc = 1;
                                      ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:380:35: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.fp_sdcc = 1;
                                      ^ ~

Fix that by using "unsigned int" as the bitfield's underlying type.

(cherry picked from commit 07f285934886016ddd82cac99a3873e68b499d5c)

Change-Id: I3550a0112f993865dc70b18f02ab11bb5012693d
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30423
---
 gdb/z80-tdep.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/gdb/z80-tdep.c b/gdb/z80-tdep.c
index aa296c7169f6..2af6198ff3fc 100644
--- a/gdb/z80-tdep.c
+++ b/gdb/z80-tdep.c
@@ -98,11 +98,11 @@ struct z80_unwind_cache
 
   struct
   {
-    int called:1;	/* there is return address on stack */
-    int load_args:1;	/* prologues loads args using POPs */
-    int fp_sdcc:1;	/* prologue saves and adjusts frame pointer IX */
-    int interrupt:1;	/* __interrupt handler */
-    int critical:1;	/* __critical function */
+    unsigned int called : 1;    /* there is return address on stack */
+    unsigned int load_args : 1; /* prologues loads args using POPs */
+    unsigned int fp_sdcc : 1;   /* prologue saves and adjusts frame pointer IX */
+    unsigned int interrupt : 1; /* __interrupt handler */
+    unsigned int critical : 1;  /* __critical function */
   } prologue_type;
 
   /* Table indicating the location of each and every register.  */
-- 
2.40.1