GCC Translation Validation

I am planning to do some work with SMT solvers and GCC. I usually start new projects by doing a naive implementation of the critical part to get a feel for the problems and find out what I need to learn before the real implementation. So I started this project by building a simple translation validator, similar to the LLVM Alive2 (but with many limitations).

My implementation, pysmtgcc, is rather limited, but has already found a few bugs in GCC1 by running the tool on the c-c++-common, gcc.c-torture, gcc.dg, and gcc.misc-tests tests suites.

What the tool does

The tool takes GIMPLE IR for two functions and checks that the second is a refinement of the first. That is,

• The returned value is the same for both functions for all input that does not invoke undefined behavior.
• The second does not have any undefined behavior that the first does not have.
• The global memory is the same after invoking both functions with input that does not invoke undefined behavior.

The functionality is implemented as GCC python plugins: plugin1.py and plugin2.py.2

plugin1.py

plugin1.py compares the IR before/after each GIMPLE pass3 and complains if the resulting IR is not a refinement of the input (that is, if the GIMPLE pass miscompiled the program).

For example, compiling the function f7 from PR 106523

unsigned char f7(unsigned char x, unsigned int y)
{
unsigned int t = x;
return (t << y) | (t >> ((-y) & 7));
}


with a compiler where PR 106523 is not fixed (for example, GCC 12.2) using plugin1.py

gcc -O3 -fno-strict-aliasing -c -fplugin=python -fplugin-arg-python-script=plugin1.py pr106523.c


gives us the output

pr106523.c: In function 'f7':
pr106523.c:1:15: note: Transformation ccp -> forwprop is not correct (retval).
1 | unsigned char f7(unsigned char x, unsigned int y)
|               ^~
pr106523.c:1:15: note: [y = 13, x = 198]
src retval: 24
tgt retval: 216


telling us that the forwprop4 pass miscompiled the function so it now returns 216 instead of 24 when called as f7(198, 13).

plugin2.py

plugin2.py requires the translation unit to consist of two functions named src and tgt, and it verifies that tgt is a refinement of src.

For example, testing changing the order of signed addition

int src(int a, int b, int c)
{
return a + c + b;
}

int tgt(int a, int b, int c)
{
return a + b + c;
}


by compiling as

gcc -O3 -fno-strict-aliasing -c -fplugin=python -fplugin-arg-python-script=plugin2.py example.c


gives us the output

example.c: In function 'tgt':
example.c:6:5: note: Transformation is not correct (UB).
6 | int tgt(int a, int b, int c)
|     ^~~
example.c:6:5: note: [c = 1793412222, a = 3429154139, b = 2508144171]


telling us that tgt invokes undefined behavior in cases where src does not, and gives us an example of input where this happen (the values are, unfortunately, written as unsigned values. In this case, it means [c = 1793412222, a = -865813157, b = -1786823125]).

Note: plugin2.py works on the IR from the ssa pass, i.e., early enough that the compiler has not done many optimizations. But GCC does peephole optimizations earlier (even when compiling as -O0), so you cannot use this plugin to test such optimizations. It is good practice to check with -fdump-tree-ssa that the IR used by the tool looks as expected.

Limitations

Some of the major limitations in the current version:

• Function calls are not implemented.
• Loops are not implemented.
• The CONSTRUCTOR tree-code is ignored5. This is used for brace-enclosed initializers for a structure or an array, which means that the tool will report bogus errors for most programs initializing structures and arrays.
• Support for memory operations is a bit shaky:
• The tool is not tracking uninitialized memory.
• It is confused about what memory can be pointed to by global pointers.
• malloc etc. are not supported.
• The tool often reports spurious memory-related errors unless -fno-strict-aliasing is passed to the compiler.
• Pointer size/memory order is hardcoded as 64 bits/little-endian.

Another annoying limitation is that GCC is doing folding (i.e., peephole optimizations) before running GIMPLE passes, so the tool will not find bugs in that code. Alive/Alive2 found several bugs in the LLVM equivalent instcombine pass, so it is likely that GCC also has bugs in its peephole optimizations.

How pysmtgcc works

I will follow up this blog post with a series of posts describing how this works, design decisions, and what I have learned. Tentative outline:

1. Writing a GCC plugin and basics of GCC GIMPLE IR.
2. Using SMT solvers for translation validation. More about GIMPLE semantics.
3. Control flow.
4. Memory.

Further work

I plan to spend some time improving the memory handling, but then I’ll declare this experiment to be “done” and start preparing for a better implementation (using C++ instead of Python).

1. Naming is hard.

2. Some passes are not checked as they do things that the tool cannot follow; mostly IPA passes, but also passes such as esra that give us spurious errors because of limitations in the current memory implementation.

3. The message says “ccp -> forwprop” where ccp is the pass before the offending pass. This makes it easier to identify which execution misbehaved for passes that are run multiple times (the python API does not give us the pass number used in GCC dumps).

4. This is because CONSTRUCTOR is not fully implemented in gcc-python-plugin – it gives us an object without any data. We could just report “not implemented”, but that would make most tests fail to analyze as CONSTRUCTOR is also used to report the end of scope for memory objects.

Written on September 13, 2022