8000 Multiple unused code warnings in `Python/generated_cases.c.h` · Issue #125515 · python/cpython · GitHub
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sobolevn opened this issue Oct 15, 2024 · 3 comments
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Multiple unused code warnings in Python/generated_cases.c.h #125515

sobolevn opened this issue Oct 15, 2024 · 3 comments
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@markshannon
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build The build process and cross-build interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error

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@sobolevn
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sobolevn commented Oct 15, 2024 •
edited by bedevere-app bot
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Bug report

Link: https://github.com/python/cpython/actions/runs/11333284963/job/31517440086#step:6:1025

Report:

 In file included from Python/ceval.c:870:
Python/generated_cases.c.h:5062:13: warning: code will never be executed [-Wunreachable-code]
            stack_pointer += -1;
            ^~~~~~~~~~~~~
Python/generated_cases.c.h:4748:13: warning: code will never be executed [-Wunreachable-code]
            stack_pointer += -1;
            ^~~~~~~~~~~~~
Python/generated_cases.c.h:803:31: warning: code will never be executed [-Wunreachable-code]
clang -c -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -D_Py_TIER2=1 -D_Py_JIT -flto=thin -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden -fprofile-instr-generate -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Python/codegen.o Python/codegen.c
                for (int _i = oparg; --_i >= 0;) {
                              ^~~~~
Python/generated_cases.c.h:689:31: warning: code will never be executed [-Wunreachable-code]
                for (int _i = oparg*2; --_i >= 0;) {
                              ^~~~~

This looks like we have two different problems:

  1. stack manipulations are added after goto and return:

    cpython/Python/generated_cases.c.h

    Lines 4747 to 4749 in cc5a225

    GO_TO_INSTRUCTION(LOAD_SUPER_ATTR);
    stack_pointer += -1;
    assert(WITHIN_STACK_BOUNDS());
    and

    cpython/Python/generated_cases.c.h

    Lines 5061 to 5063 in cc5a225

    return PyStackRef_AsPyObjectSteal(retval);
    stack_pointer += -1;
    assert(WITHIN_STACK_BOUNDS());
  2. some logical? problem with the loop definition

CC @Fidget-Spinner

Linked PRs
@sobolevn sobolevn added type-bug An unexpected behavior, bug, or error interpreter-core (Objects, Python, Grammar, and Parser dirs) labels Oct 15, 2024
@brandtbucher brandtbucher mentioned this issue Oct 15, 2024
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@markshannon markshannon self-assigned this Oct 16, 2024
@bedevere-app bedevere-app bot mentioned this issue Oct 18, 2024
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@markshannon
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I'm not sure what we can do about the compiler compiling about

if (CONVERSION_FAILED(x)) {
   ... // compiler complains this is unreachable
}

as CONVERSION_FAILED(x) is #defined as 0 in the with-gil build.

markshannon added a commit that referenced this issue Oct 28, 2024
@markshannon
GH-125515: Reduce number of compiler warnings in generated code (GH-1…
2544159 
…25697)
picnixz pushed a commit to picnixz/cpython that referenced this issue Dec 8, 2024
@markshannon @picnixz
pythonGH-125515: Reduce number of compiler warnings in generated code (…
574ec0e 
…pythonGH-125697)
ebonnal pushed a commit to ebonnal/cpython that referenced this issue Jan 12, 2025
@markshannon @ebonnal
pythonGH-125515: Reduce number of compiler warnings in generated code (…
839737f 
…pythonGH-125697)
@picnixz
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picnixz commented Apr 25, 2025 •
edited
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Is there anything else to do here? I actually have new warnings but I'm unsure whether you want to be them part of this issue:

In file included from Python/ceval.c:1097:
Python/generated_cases.c.h:12157:13: warning: code will never be executed [-Wunreachable-code]
 12157 |             stack_pointer -= 3;
       |             ^~~~~~~~~~~~~
Python/generated_cases.c.h:12150:13: warning: code will never be executed [-Wunreachable-code]
 12150 |             stack_pointer -= 4;
       |             ^~~~~~~~~~~~~
Python/generated_cases.c.h:7462:13: warning: code will never be executed [-Wunreachable-code]
 7462 |             stack_pointer += -1;
      |             ^~~~~~~~~~~~~

I'm using clang-18 and the following configure:

./configure --with-undefined-behavior-sanitizer --prefix="$(pwd)/build" CC=clang LD=clang CFLAGS="-fsanitize=undefined -fno-sanitize-recover"  LDFLAGS="-fsanitize=undefined -fno-sanitize-recover" -q

@picnixz picnixz added the build The build process and cross-build label Apr 25, 2025
@freakboy3742
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From a conversation with @brandtbucher, the first two can be resolved by commenting out the pop_4_error and pop3_error cases in bytecodes.c.

The last error is more complicated; the offending block is this one; the complication is that it's the only opcode that can return from inside the instruction. The "unreachable code" is the automatically injected content that is done for cleanup after the return. The suggestion was that the code generator needs to be taught about that edge case so it doesn't inject the stack cleanup code.

@bedevere-app bedevere-app bot mentioned this issue Apr 29, 2025
Merged
freakboy3742 pushed a commit that referenced this issue Apr 30, 2025
@brandtbucher
GH-125515: Mark code after "return" as unreachable in the cases gener…
b329096 
…ator (#133178)

Mark code after "return" as unreachable
@bedevere-app bedevere-app bot mentioned this issue Apr 30, 2025
Merged
freakboy3742 added a commit that referenced this issue Apr 30, 2025
@freakboy3742
GH-125515: Remove two unused error branches. (#133181)
6c522de 
Remove two unused error branches in the generated bytecode handling.
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