dump-c: sign-extend signed bit-field temporaries

[tautschnig]

When goto-conversion lowers a chained assignment outer = bf = expr through a bit-field lvalue, it introduces a temporary symbol whose declared type is the bit-field type. goto_program2code rewrites that temporary's type to the bit-field's underlying integer type and previously masked the initialiser with & ((1 << N) - 1) to model the bit-field truncation.

For unsigned bit-fields the AND-mask is enough, but for signed bit-fields the value of bf = expr is the value of bf after the assignment - i.e. the masked value sign-extended to the underlying type (per C11 6.5.16.1p3). Without the sign extension, the temporary held an unsigned-truncated value, so subsequent uses of it (here: the chained assignment to outer) observed the wrong value. The original program continued to verify correctly under CBMC, but the C code emitted by --dump-c no longer agreed with the source - which CSmith picks up as a checksum mismatch.

Apply branchless sign extension ((v & mask) ^ sign_bit) - sign_bit for the signed-and-narrower case. This expression stays within the bit-field's value range and so triggers neither signed overflow nor implementation-defined shift behaviour in the dumped C.

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[Copilot]

Pull request overview

This PR fixes --dump-c output semantics for chained assignments through signed bit-fields by ensuring that temporaries introduced during lowering observe the C-standard-required sign-extended value of the bit-field after assignment (C11 6.5.16.1p3). This aligns dumped C with source behavior and prevents checksum mismatches (e.g., from CSmith) while leaving verification behavior unchanged.

Changes:

• Update goto_program2code bit-field temporary rewriting to apply branchless sign extension for signed, narrower-than-underlying bit-fields.
• Keep existing masking behavior for unsigned bit-fields and for signed bit-fields that are not narrower than their underlying type.
• Add a regression test that exercises outer = s.r = v where s.r is a 2-bit signed bit-field and checks the dumped-C path preserves sign-extension semantics.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/goto-instrument/goto_program2code.cpp	Sign-extends masked initializer for signed, narrow bit-field temporaries when rewriting their declared type to the underlying integer type.
regression/goto-instrument/dump-bitfield-chained-assign/test.desc	Adds a --dump-c regression test description to ensure dumped C remains verifiable and preserves correct chained-assignment semantics.
regression/goto-instrument/dump-bitfield-chained-assign/main.c	Minimal C program reproducing the signed bit-field chained-assignment sign-extension case (2 stored into 2-bit signed bit-field yields -2).

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[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.55%. Comparing base (808dae2) to head (17fdcb6).

Files with missing lines	Patch %	Lines
src/goto-instrument/goto_program2code.cpp	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9013   +/-   ##
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  Coverage    80.55%   80.55%           
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  Files         1707     1707           
  Lines       189051   189058    +7     
  Branches        73       73           
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+ Hits        152299   152305    +6     
- Misses       36752    36753    +1     

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