[RFA] Fix the clang build

[RFA] Fix the clang build

[Tom Tromey]

Simon pointed out that gdb would not build with clang, due to the
addition of -Wimplicit-fallthrough.  This patch fixes the problem by
using -Wimplicit-fallthrough=3 -- this does not work with clang,
bypassing the issue.

Tested by rebuilding with both gcc and clang; and also by verifying
that -Wimplicit-fallthrough=3 is used in the gcc build.

I will file a follow-up bug to convert the fall-through comments to a
form that can be used by both clang and gcc.

gdb/ChangeLog
2018-05-10  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* warning.m4 (AM_GDB_WARNINGS): Use -Wimplicit-fallthrough=3.

gdb/gdbserver/ChangeLog
2018-05-10  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
---
 gdb/ChangeLog           | 5 +++++
 gdb/configure           | 2 +-
 gdb/gdbserver/ChangeLog | 4 ++++
 gdb/gdbserver/configure | 2 +-
 gdb/warning.m4          | 2 +-
 5 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/gdb/configure b/gdb/configure
index 0eec780bd1..4186f7e160 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -15367,7 +15367,7 @@ build_warnings="-Wall -Wpointer-arith \
 -Wno-mismatched-tags \
 -Wno-error=deprecated-register \
 -Wsuggest-override \
--Wimplicit-fallthrough \
+-Wimplicit-fallthrough=3 \
 -Wduplicated-cond"
 
 case "${host}" in
diff --git a/gdb/gdbserver/configure b/gdb/gdbserver/configure
index ac61ba3543..b2e2e8f955 100755
--- a/gdb/gdbserver/configure
+++ b/gdb/gdbserver/configure
@@ -7174,7 +7174,7 @@ build_warnings="-Wall -Wpointer-arith \
 -Wno-mismatched-tags \
 -Wno-error=deprecated-register \
 -Wsuggest-override \
--Wimplicit-fallthrough \
+-Wimplicit-fallthrough=3 \
 -Wduplicated-cond"
 
 case "${host}" in
diff --git a/gdb/warning.m4 b/gdb/warning.m4
index cb23472ec6..632cc214ac 100644
--- a/gdb/warning.m4
+++ b/gdb/warning.m4
@@ -44,7 +44,7 @@ build_warnings="-Wall -Wpointer-arith \
 -Wno-mismatched-tags \
 -Wno-error=deprecated-register \
 -Wsuggest-override \
--Wimplicit-fallthrough \
+-Wimplicit-fallthrough=3 \
 -Wduplicated-cond"
 
 case "${host}" in
-- 
2.13.6

Re: [RFA] Fix the clang build

[Simon Marchi]

On 2018-05-10 10:40, Tom Tromey wrote:
> Simon pointed out that gdb would not build with clang, due to the
> addition of -Wimplicit-fallthrough.  This patch fixes the problem by
> using -Wimplicit-fallthrough=3 -- this does not work with clang,
> bypassing the issue.
> 
> Tested by rebuilding with both gcc and clang; and also by verifying
> that -Wimplicit-fallthrough=3 is used in the gcc build.
> 
> I will file a follow-up bug to convert the fall-through comments to a
> form that can be used by both clang and gcc.

Thanks, that patch is fine with me.

To be clear for those who might not be aware of the mechanism: the 
configure script tests each warning flag to see if it's supported by the 
current compiler.  Since -Wimplicit-fallthrough=3 is not supported by 
clang, it won't be included.  This means that we won't get the 
fallthrough warnings when building with clang.  I think it's fine, since 
we get them through gcc, so an unintended fallthrough would be found 
pretty quickly anyway.

Simon