From 493e9dd8c6f49a4738e4434152728b4a465f9ec4 Mon Sep 17 00:00:00 2001
From: Peter Bex <address@hidden>
Date: Mon, 16 Oct 2017 20:35:33 +0200
Subject: [PATCH 2/2] Fix flonum to string conversion on extreme edge cases on
 64-bit archs

This bug was exposed by the previous commit which restored the tests
to work as intended, because the serialization of (expt 2.0 64)
resulted in "0.0" in compiled code, causing the exact/inexact
comparison tests between the bignum (expt 2 64) and the flonum
(expt 2.0 64.0) to fail.

This representation fails because C_fits_in_unsigned_int_p will
incorrectly return C_SCHEME_TRUE on a 64-bit architecture due to how
we perform the casting: Because UINT_MAX is 2^64-1, it will be
normalized by the floating-point representation to the nearest
floating-point value that can represent it: 2^64.
Thus, (double)2^64 <= UINT_MAX is true.  When we then cast the
double to an unsigned word, it will result in zero due to overflow.

The fix here is to simply check if the log2 of the number is less than
the number of bits in an UWORD, rather than trying a direct value
comparison.  As a bonus, this allows us to finally get rid of the
obsolete C_fits_in_unsigned_int_p() inline helper function.
---
 chicken.h | 14 --------------
 runtime.c |  4 ++--
 2 files changed, 2 insertions(+), 16 deletions(-)

diff --git a/chicken.h b/chicken.h
index 9e2af974..45571d45 100644
--- a/chicken.h
+++ b/chicken.h
@@ -2420,20 +2420,6 @@ inline static C_word C_i_bignump(C_word x)
 }
 
 
-
-/* XXX TODO OBSOLETE (but still used by C_flonum_to_string) */
-inline static C_word C_fits_in_unsigned_int_p(C_word x)
-{
-  double n, m;
-
-  if(x & C_FIXNUM_BIT) return C_SCHEME_TRUE;
-  if(C_truep(C_i_bignump(x))) return C_mk_bool(C_bignum_size(x) == 1);
-
-  n = C_flonum_magnitude(x);
-  return C_mk_bool(C_modf(n, &m) == 0.0 && n >= 0 && n <= C_UWORD_MAX);
-}
-
-
 inline static double C_c_double(C_word x)
 {
   if(x & C_FIXNUM_BIT) return (double)C_unfix(x);
diff --git a/runtime.c b/runtime.c
index e1970cd6..2997b1a5 100644
--- a/runtime.c
+++ b/runtime.c
@@ -10432,7 +10432,7 @@ void C_ccall C_fixnum_to_string(C_word c, C_word *av)
 void C_ccall C_flonum_to_string(C_word c, C_word *av)
 {
   C_char *p;
-  double f;
+  double f, m;
   C_word *a,
     /* self = av[ 0 ] */
     k = av[ 1 ],
@@ -10450,7 +10450,7 @@ void C_ccall C_flonum_to_string(C_word c, C_word *av)
     barf(C_BAD_ARGUMENT_TYPE_BAD_BASE_ERROR, "number->string", C_fix(radix));
   }
 
-  if(C_fits_in_unsigned_int_p(num) == C_SCHEME_TRUE) { /* Use fast int code */
+  if(C_modf(f, &m) == 0.0 && f >= 0 && log2(f) < C_WORD_SIZE) { /* Use fast int code */
     if(f < 0) {
       p = to_n_nary((C_uword)-f, radix, 1, 1);
     } else {
-- 
2.11.0