Multiple unused code warnings in `Python/generated_cases.c.h`

[sobolevn]

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

• GH-125515: Reduce number of compiler warnings in generated code #125697
• GH-125515: Mark code after "return" as unreachable in the cases generator #133178
• GH-125515: Remove two unused error branches. #133181

[markshannon]

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.

[picnixz]

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

[freakboy3742]

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.