Fix ROUND_TO_AWAY rounding decision in float_bvt

[tautschnig]

float_bvt::fraction_rounding_decision used
or_exprt(rounding_bit, sticky_bit)
for ROUND_TO_AWAY, which incorrectly rounds away from zero whenever any extra bits are non-zero. The correct formula is just rounding_bit, matching float_utilst.

ROUND_TO_AWAY means 'round to nearest, ties away from zero':

• rounding_bit=0: closer to truncated value, don't increment
• rounding_bit=1, sticky=0: exact tie, round away (increment)
• rounding_bit=1, sticky=1: closer to incremented value, increment

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[Copilot]

Pull request overview

Fixes the ROUND_TO_AWAY (round-to-nearest, ties away from zero) rounding increment decision in the float_bvt encoding to match IEEE-754 semantics and float_utilst, and adds a regression test for both SAT (--floatbv) and SMT2/Z3 paths.

Changes:

• Correct float_bvt::fraction_rounding_decision for ROUND_TO_AWAY to use only rounding_bit (not rounding_bit || sticky_bit).
• Add a new regression test Float-round-to-away covering tie and non-tie cases.
• Add an SMT2/Z3 regression descriptor to exercise the SMT path.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/solvers/floatbv/float_bv.cpp	Fixes the rounding increment condition for ROUND_TO_AWAY in the floatbv encoding.
regression/cbmc/Float-round-to-away/main.c	Adds a targeted regression program for ROUND_TO_AWAY tie/non-tie behavior.
regression/cbmc/Float-round-to-away/test.desc	Runs the regression with --floatbv (SAT/bit-vector path).
regression/cbmc/Float-round-to-away/test_smt.desc	Runs the regression with --smt2 --z3 (SMT path).

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[Copilot]

The comment says 1.5 * 1.1 = 1.65 exactly, but 1.1f is not exactly representable as a float, so the floating-point multiplication operands are already rounded. Consider rewording to clarify this is exact in real arithmetic, but not in IEEE-754 float (or use hex-float literals throughout if you want exact operand values).

Suggested change

	// 1.5 * 1.1 = 1.65 exactly. In float: 0x1.a66666p+0 (1.6499999...).
	// Same as ROUND_TO_EVEN since it's not a tie.
	// In real arithmetic, 1.5 * 1.1 = 1.65 exactly, but 1.1f is not exactly
	// representable in binary32, so the float operands are already rounded.
	// This multiplication is still a non-tie case, so ROUND_TO_AWAY should
	// match ROUND_TO_EVEN.

[Copilot]

This test invokes the SMT solver (--smt2 --z3) but the label line doesn’t include smt-backend. In this repo, the default cbmc regression run excludes tests tagged smt-backend (see regression/cbmc/CMakeLists.txt), so SMT-only tests are typically labeled accordingly. Consider adding the smt-backend tag to avoid running this in SAT-only profiles / environments without Z3.

Suggested change

	CORE no-new-smt
	CORE no-new-smt smt-backend

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.55%. Comparing base (a6531d4) to head (3e0b040).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8983      +/-   ##
===========================================
+ Coverage    80.50%   80.55%   +0.05%     
===========================================
  Files         1704     1704              
  Lines       188796   188796              
  Branches        73       73              
===========================================
+ Hits        151993   152092      +99     
+ Misses       36803    36704      -99     

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