Unreachable code warnings while building latest main at 48c70b8

Bug report

Bug description:

I ran the ./configure and make, here are the warnings shown.

gcc -c -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Python/ast.o Python/ast.c
gcc -c -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Python/ast_opt.o Python/ast_opt.c
gcc -c -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Python/ast_unparse.o Python/ast_unparse.c
gcc -c -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Python/bltinmodule.o Python/bltinmodule.c
gcc -c -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Python/brc.o Python/brc.c
gcc -c -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Python/ceval.o Python/ceval.c
In file included from Python/ceval.c:904:
Python/generated_cases.c.h:808:31: warning: code will never be executed [-Wunreachable-code]
  808 |                 for (int _i = oparg; --_i >= 0;) {
      |                               ^~~~~
Python/generated_cases.c.h:694:31: warning: code will never be executed [-Wunreachable-code]
  694 |                 for (int _i = oparg*2; --_i >= 0;) {
      |                               ^~~~~
2 warnings generated.

Operating System:

➜  cpython git:(main) uname -a
Darwin SREDDY- 24.1.0 Darwin Kernel Version 24.1.0: Thu Oct 10 21:02:27 PDT 2024; root:xnu-11215.41.3~2/RELEASE_X86_64 x86_64

Main branch commit : 48c70b8

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

I have updated the tcl-tk to version 9.0.0_1, the warnings are gone. Please refer - build.log

I cannot reproduce this on my MacBook. The tcl-tk comment seems off-topic for this issue. Can you provide more information about your environment?

Like gcc compiler version? and ...?

Include everything that is needed to reproduce the failure you are seeing. Are you still seeing these failures? Did you compile from a clean source tree (git clean -fdx)?

➜  cpython git:(main) env
TERM_SESSION_ID=REDACTED
SSH_AUTH_SOCK=/private/tmp/com.apple.launchd.xFPWqTHwrg/Listeners
LC_TERMINAL_VERSION=3.5.10
COLORFGBG=7;0
ITERM_PROFILE=REDACTED
XPC_FLAGS=0x0
LANG=en_US.UTF-8
PWD=/Users/REDACTED/workspace/github_repos/cpython
SHELL=/bin/zsh
__CFBundleIdentifier=com.googlecode.iterm2
TERM_FEATURES=REDACTED
TERM_PROGRAM_VERSION=3.5.10
TERM_PROGRAM=iTerm.app
PATH=/Users/REDACTED/.nvm/versions/node/v20.17.0/bin:/Library/Frameworks/Python.framework/Versions/3.8/bin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Library/Apple/usr/bin:/Users/REDACTED/.cargo/bin:/Applications/iTerm.app/Contents/Resources/utilities:/Users/REDACTED/.local/bin
LC_TERMINAL=iTerm2
COLORTERM=truecolor
COMMAND_MODE=unix2003
TERM=xterm-256color
TERMINFO_DIRS=/Applications/iTerm.app/Contents/Resources/terminfo:/usr/share/terminfo
HOME=/Users/REDACTED
TMPDIR=/var/folders/6v/n8nm5d6136b0br85j6llcy3c0000gn/T/
USER=REDACTED
XPC_SERVICE_NAME=0
LOGNAME=REDACTED
ITERM_SESSION_ID=REDACTED
__CF_USER_TEXT_ENCODING=0x0:0:0
SHLVL=1
OLDPWD=/Users/REDACTED/workspace/github_repos
ZSH=/Users/REDACTED/.oh-my-zsh
PAGER=less
LESS=-R
LSCOLORS=Gxfxcxdxbxegedabagacad
NVM_DIR=/Users/REDACTED/.nvm
NVM_CD_FLAGS=-q
NVM_BIN=/Users/REDACTED/.nvm/versions/node/v20.17.0/bin
NVM_INC=/Users/REDACTED/.nvm/versions/node/v20.17.0/include/node
LC_ALL=en_US.UTF-8
_=/usr/bin/env

➜  cpython git:(main) sw_vers
ProductName:		macOS
ProductVersion:		15.1.1
BuildVersion:		24B91

➜  cpython git:(main) gcc --version
Apple clang version 16.0.0 (clang-1600.0.26.6)
Target: x86_64-apple-darwin24.1.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

➜  cpython git:(main) xcodebuild -version
Xcode 16.2
Build version 16C5032a

➜  cpython git:(main) clang --version
Apple clang version 16.0.0 (clang-1600.0.26.6)
Target: x86_64-apple-darwin24.1.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

Still unable to reproduce. I'm on the same macOS version and the same compiler.

Here is the ./configure output i have captured.

config.log

Here is the make output i have capture with the command - make &> make.log. You should be able to see the warnings in the make.log file.

make.log

In file included from Python/ceval.c:904:
Python/generated_cases.c.h:808:31: warning: code will never be executed [-Wunreachable-code]
  808 |                 for (int _i = oparg; --_i >= 0;) {
      |                               ^~~~~
Python/generated_cases.c.h:694:31: warning: code will never be executed [-Wunreachable-code]
  694 |                 for (int _i = oparg*2; --_i >= 0;) {
      |                               ^~~~~
2 warnings generated.

Main branch at b4f799b

Try to also do a make distclean, make clean and make cleantest. Some files might be left.

Actually, to be on the safe side, please do the following, then build again:

$ git reset --hard HEAD
$ git clean -fdx

FTR: git clean -fdx would also remove all files that you manually added to the CPython project but are not in .gitignore and not committed (I have some) so I'd advise using a different directory than the directory you're usually using for development.

I also see unreachable-code warnings in generated_cases.c.h but only when not using --with-pydebug.

FTR, using the current HEAD of main using Apple-supplied clang. Environment is current macOS and Xcode releases. Also same warnings with a slightly older release of Xcode and clang.

$ git show
commit 1153e66e20124b8f3484bcaddbc0e252d31161a6 (HEAD -> main, upstream/main, upstream/HEAD)
Author: Victor Stinner <vstinner@python.org>
Date:   Tue Jan 14 11:18:52 2025 +0100

    gh-109959: Skip test_glob.test_selflink() flaky test (#128812)  
[...]
$ clang --version
Apple clang version 16.0.0 (clang-1600.0.26.6)
Target: arm64-apple-darwin24.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
$ xcodebuild -version
Xcode 16.2
Build version 16C5032a
$ sw_vers
ProductName:		macOS
ProductVersion:		15.2
BuildVersion:		24C101
$ CC=clang ./configure && make -j4
[...]
clang -c -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden -Werror=unguarded-availability  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Python/ceval.o Python/ceval.c
In file included from Python/ceval.c:906:
Python/generated_cases.c.h:4489:34: warning: code will never be executed [-Wunreachable-code]
                    for (int i = 0; i < total_args; i++) {
                                 ^
Python/generated_cases.c.h:965:34: warning: code will never be executed [-Wunreachable-code]
                    for (int i = 0; i < total_args; i++) {
                                 ^
Python/generated_cases.c.h:824:31: warning: code will never be executed [-Wunreachable-code]
                for (int _i = oparg; --_i >= 0;) {
                              ^~~~~
Python/generated_cases.c.h:710:31: warning: code will never be executed [-Wunreachable-code]
                for (int _i = oparg*2; --_i >= 0;) {
                              ^~~~~

@markshannon

cc @markshannon

Those are preceded by if (CONVERSION_FAILED(...)) which expands to if (0) in a "regular GIL" build, i.e. when Py_GIL_DISABLED is not defined. Interestingly, there are more such ifs, but not in all cases the warning is emitted for the following code line.

I think like unused labels are suppressed in generated code, these should be suppressed, too.
I've added # pragma GCC diagnostic ignored "-Wunreachable-code" to

cpython/Python/ceval.c

Lines 725 to 727 in f26daa9

    
           #if defined(__GNUC__) 
        
           #  pragma GCC diagnostic push 
        
           #  pragma GCC diagnostic ignored "-Wunused-label"

and also added || defined(__clang_) to get rid of this warning in clang-cl based builds ...

For clang, can't we add #pragma clang diagnostic? Also, I believe we should patch Tools/generator_cases.py instead of ceval.c as this is the script responsible for generating Python/generated_cases.c.h and only patch it locally and not make a global #pragma (but I don't know if it's feasible)

"Regular" clangs react on __GNUC__, I only added __clang__ because I did not want to see those warnings in Windows using clang-cl, which does (must) not define __GNU__.

I agree that patching Tools/generator_cases.py would be even better if someone wants to bite that bullet :)

I’m still able to reproduce the warnings with this workflow, which effectively eliminates the possibility that any custom settings or files are causing interference.

git clone https://github.com/python/cpython.git cpython_dev --depth=1
cd cpython_dev
./configure
make

At HEAD == 5dbd27d I get none of the "unreachable code" warnings mentioned in the OP. ~~I do get one regarding the newly added zstd module, though (fixing that one via the zstd issue: #133947)~~¹. Can we close this as out-of-date?

fix merged ↩

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Unreachable code warnings while building latest main at 48c70b8 #128086

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	#if defined(__GNUC__)
	# pragma GCC diagnostic push
	# pragma GCC diagnostic ignored "-Wunused-label"

Unreachable code warnings while building latest main at 48c70b8 #128086

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

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srinivasreddy commented on Dec 19, 2024

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