Re: [PATCH 03/11] Rewrite find_cxx_header config configure.ac

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 29/10/22 01:03, Fangrui Song wrote:
> On 2022-10-28, Adhemerval Zanella via Libc-alpha wrote:
>> clang does not support -MP option to create phony target for each
>> dependency other than the main file.  Use -fsyntax-only with a
>> more comprensible regex to get the the cxx header.
> 
> Clang has supported -MP for many years.
> The problem was that when the main file is <stdin>, the first non-main-file dependency is not listed
> in the -MP output.
> 
>     echo "#include <cstdlib>" | fclang++ -M -MP -x c++ - 2>/dev/null | sed -n "\,cstdlib:,{s/:\$//;p}"
> 
> I just fixed this in https://github.com/llvm/llvm-project/commit/ff9576f74514b836e1ba0268409a2ecb919d7118
> which shall be included in Clang 16.

You are right, I did not write down my finding and I have to hasty
(and wrongly) described the patch.


> 
> 
> 
>> ---
>> configure    | 4 ++--
>> configure.ac | 4 ++--
>> 2 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/configure b/configure
>> index ff2c406b3b..26bd8200dd 100755
>> --- a/configure
>> +++ b/configure
>> @@ -5579,8 +5579,8 @@ fi
>> # copy of those headers in Makerules.
>> if test -n "$CXX"; then
>>   find_cxx_header () {
>> -    echo "#include <$1>" | $CXX -M -MP -x c++ - 2>/dev/null \
>> -     | sed -n "\,$1:,{s/:\$//;p}"
>> +    echo "#include <$1>" | $CXX -H -fsyntax-only -x c++ - 2>&1 \
>> +     | sed -rn "\,^\.? .,{s/^\.*\. //p}"
> 
> That said, the patch looks good to support more Clang versions.
> 
> For portability, prefer -E to -r and add `;` before `}` (FreeBSD sed doesn't allow `p}`).
> 
> Actually no ERE is needed and   sed -n "\,^\. .,{s/^\. //;p;}"  should work.
> Alternatively, use      awk '$1 == "."{print $2}'

The awk does seems to be simpler solution indeed.

> 
>>   }
>>   CXX_CSTDLIB_HEADER="$(find_cxx_header cstdlib)"
>>   CXX_CMATH_HEADER="$(find_cxx_header cmath)"
>> diff --git a/configure.ac b/configure.ac
>> index eb5bc6a131..a009e7a17f 100644
>> --- a/configure.ac
>> +++ b/configure.ac
>> @@ -1136,8 +1136,8 @@ AC_SUBST(CXX_SYSINCLUDES)
>> # copy of those headers in Makerules.
>> if test -n "$CXX"; then
>>   find_cxx_header () {
>> -    echo "#include <$1>" | $CXX -M -MP -x c++ - 2>/dev/null \
>> -     | sed -n "\,$1:,{s/:\$//;p}"
>> +    echo "#include <$1>" | $CXX -H -fsyntax-only -x c++ - 2>&1 \
>> +     | sed -rn "\,^\.? .,{s/[^\.]*\. //p}"
>>   }
>>   CXX_CSTDLIB_HEADER="$(find_cxx_header cstdlib)"
>>   CXX_CMATH_HEADER="$(find_cxx_header cmath)"
>> -- 
>> 2.34.1
>>