[mypyc] declare Py_ssize_t i outside of loop initialization

[thomasjohns]

Description

Fixes mypyc/mypyc#800

I believe this is the only instance of a variable declaration inside a for-loop initialization. Removing this may help with consistent compilation across different C compiler configurations. See linked issue for more details.

[TH3CHARLie]

Thanks for finding this and fixing it!

I'd say there should be some more systematic way to fix this but this definitely helps. Can you confirm the error reported in your issue getting solved with this PR?(I cannot reproduce on my machine).

[thomasjohns]

Hi @TH3CHARLie thanks for your question. Yes for me with latest master, any program will not compile when running scripts/mypyc program.py with:

/mnt/c/Users/thoma/Code/mypy/mypyc/lib-rt/getargsfast.c:99:9: error: ‘for’ loop initial declarations are only allowed in C99 mode
         for (Py_ssize_t i = 0; i < nargs; i++) {
         ^
/mnt/c/Users/thoma/Code/mypy/mypyc/lib-rt/getargsfast.c:99:9: note: use option -std=c99 or -std=gnu99 to compile your code

However, with this patch mypyc compatible programs do compile with scripts/mypyc program.py since the previous "error: 'for' loop initial decl..." isn't raised.

---

After a bit more experimentation, I found that I get this compiler error with gcc version 4.8.5 and I don't get this compiler error on latest master with gcc version 7.5.0 and clang version 3.4. So this was all likely due to me having an old version of gcc on my system.

One way to solve this generally (if we want to be able to use C code such as for (int i = 0; ...) might be to add certain flags to mypyc/build.py::mypycify based on detecting an old version of gcc (e.g. adding -std=c99 flag will compile with all gcc's and clang's mentioned above).

---

The following is a little noisy, but it shows what happens when I try to compile build/__native.c with various C compilers on my system (with latest master)

/m/c/U/t/C/mypy> /usr/bin/gcc --version
gcc (Ubuntu 4.8.5-4ubuntu8~14.04.2) 4.8.5
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

/m/c/U/t/C/mypy> /usr/bin/gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -I/mnt/c/Users/thoma/Code/mypy/mypyc/lib-rt -I/mnt/c/Users/thoma/Code/mypy/.venv/include -I/usr/local/include/python3.9 -c build/__native.c -o build/temp.linux-x86_64-3.9/build/__native.o -O3 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable
In file included from build/__native.c:3:0:
/mnt/c/Users/thoma/Code/mypy/mypyc/lib-rt/getargsfast.c: In function ‘CPyArg_ParseStackAndKeywordsSimple’:
/mnt/c/Users/thoma/Code/mypy/mypyc/lib-rt/getargsfast.c:99:9: error: ‘for’ loop initial declarations are only allowed in C99 mode
         for (Py_ssize_t i = 0; i < nargs; i++) {
         ^
/mnt/c/Users/thoma/Code/mypy/mypyc/lib-rt/getargsfast.c:99:9: note: use option -std=c99 or -std=gnu99 to compile your code
build/__native.c: At top level:
cc1: error: unrecognized command line option "-Wno-unknown-warning-option" [-Werror]
cc1: error: unrecognized command line option "-Wno-unused-command-line-argument" [-Werror]
cc1: all warnings being treated as errors

(^ error)

/m/c/U/t/C/mypy> gcc-7 --version
gcc-7 (Ubuntu 7.5.0-3ubuntu1~14.04.1) 7.5.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

/m/c/U/t/C/mypy> gcc-7 -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -I/mnt/c/Users/thoma/Code/mypy/mypyc/lib-rt -I/mnt/c/Users/thoma/Code/mypy/.venv/include -I/usr/local/include/python3.9 -c build/__native.c -o build/temp.linux-x86_64-3.9/build/__native.o -O3 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable

(^ success)

/m/c/U/t/C/mypy> clang --version
Ubuntu clang version 3.4-1ubuntu3 (tags/RELEASE_34/final) (based on LLVM 3.4)
Target: x86_64-pc-linux-gnu
Thread model: posix
/m/c/U/t/C/mypy> clang -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -I/mnt/c/Users/thoma/Code/mypy/mypyc/lib-rt -I/mnt/c/Users/thoma/Code/mypy/.venv/include -I/usr/local/include/python3.9 -c build/__native.c -o build/temp.linux-x86_64-3.9/build/__native.o -O3 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable

(^ success)

[TH3CHARLie]

One way to solve this generally (if we want to be able to use C code such as for (int i = 0; ...) might be to add certain flags to mypyc/build.py::mypycify based on detecting an old version of gcc (e.g. adding -std=c99 flag will compile with all gcc's and clang's mentioned above).

That's similar to the rough idea in my mind to solve this more systematically. Another potential(but more aggressive) alternative is to reject lower-version C compilers and document this as desired behavior.

I am merging this now. Feel free to comment below if anyone has different opinions on this