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fstar: address review comments - improve comments and table header clarity

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Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>
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4
fstar/FPARewriterRules.fst
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@ -50,7 +50,7 @@ open IEEE754
(* ================================================================== *)
(* 1a. fma(rm, ±0, y, NaN) = NaN
Source: C++ line "fma(rm, ±0, y, NaN) = NaN".
Source: C++ fpa_rewriter.cpp, mk_fma, branch "if (m_util.is_nan(arg4))".
The addend NaN propagates (IEEE 754-2019 §6.2, NaN payload rules). *)
let lemma_fma_zero_nan_addend
(#eb #sb: pos)
@ -381,7 +381,7 @@ let lemma_fma_zero_ite_nan_arm
else begin
(* is_finite y = false && is_nan y = false.
By the transparent definition is_finite y = not (is_nan y) && not (is_inf y),
Z3 can derive is_inf y = true directly. *)
the type-checker can derive is_inf y = true directly. *)
assert (is_inf y = true);
ax_zero_mul_inf rm zero_val y;
ax_fma_nan_mul rm zero_val y z

4
fstar/README.md
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@ -9,7 +9,7 @@ in the [F\*](https://www.fstar-lang.org/) proof assistant.
PR #9038 adds three families of optimizations to `src/ast/rewriter/fpa_rewriter.cpp`
that avoid expensive FP bit-blasting:
| Optimization | C++ function | Formalized in |
| Optimization | C++ function | F\* module and section |
|---|---|---|
| `fma(rm, ±0, y, z)` zero-multiplicand decomposition | `mk_fma` | `FPARewriterRules.fst` §1 |
| `isNaN/isInf/isNormal` applied to `to_fp(rm, to_real(int))` | `mk_is_nan`, `mk_is_inf`, `mk_is_normal`, `mk_is_inf_of_int` | `FPARewriterRules.fst` §2 |
@ -54,7 +54,7 @@ Derived rewrite rules proved from `IEEE754.fst` axioms.
| `lemma_fma_zero_const_nan_inf_arm` | NaN/inf arm of the same `ite` |
| `lemma_fma_zero_general_finite_arm` | finite arm for fully symbolic `y` and `z` |
| `lemma_fma_zero_general_nan_inf_arm` | NaN/inf arm for fully symbolic `y` and `z` |
| `lemma_fma_zero_product_sign` | sign of `fp_mul(rm, ±0, y)` = sign(±0) XOR sign(y) |
| `lemma_fma_zero_product_sign` | sign of `fp_mul(rm, zero_val, y)` = sign(zero_val) XOR sign(y) |
| `lemma_fma_zero_ite_correct` | full composite: `fma(rm, ±0, y, z) = ite(is_finite(y), fp_add(rm, fp_mul(rm, ±0, y), z), NaN)` |
#### Section 2 — Classification of Integer-to-Float Conversions