Implement NaN boxing.

On platforms with 64 bit pointers, and therefore 64-bit-wide
TXR values, we can use a representation technique which allows
double floating-point values to be unboxed.

Fixnum integers are reduced from 62 bits to 50, and there is
a little more complexity in the run-time type checking and
dispatch which costs extra cycles.

The support is currently off by default; it must be explicitly
enabled with ./configure --nan-boxing.

* lib.h (NUM_MAX, NUM_MIN, NUM_BIT): Define separately for
NaN boxing.
(TAG_FLNUM, TAG_WIDTH, NAN_TAG_BIT, NAN_TAG_MASK, TAG_BIGMASK,
TAG_BIGSHIFT, NAN_FLNUM_DELTA): New preprocessor symbols.
(enum type, type_t): The FLNUM enumeration constant moves
to just after LIT, so that its value is the same as TAG_FLNUM.
(struct flonum): Does not exist under NaN boxing.
(union obj): No fl member under NaN boxing.
(tag, is_ptr): Separately defined for NaN boxing.
(is_flo): New function under NaN boxing.
(tag_ex): New function. It's like tag, but identifies
floating-point values as TAG_FLNUM. The tag function continues
to map them to TAG_PTR, which is wrong under NaN boxing,
but needed in order not to separately write tons of cases in
the arith.c module.
(type): Use tag_ex, so TAG_FLNUM is handled, if it exists.
(auto_str, static_str, litptr, num_fast, chr, c_n, c_u):
Different definition for NaN boxing.
(c_ch, c_f): New function.
(throw_mismatch): Attribute with NORETURN.
(nao): Separate definition for NaN boxing.

* lib.c (seq_kind_tab): Reorder initializer to follow enum
reordering.
(seq_iter_rewind): use c_n and c_ch functions, since type
checking has been done in those cases. The self parameter
is no longer needed.
(iter_more): use c_ch on CHR object.
(equal): Use c_f accessor to get double value rather than
assuming there is a struct flonum representation.
(stringp): Use tag_ex, otherwise a floating-point number
is identified as TAG_PTR.
(diff, isec, isecp): Don't pass removed self parameter
to seq_iter_rewind.

* arith.c (c_unum, c_dbl_num, c_dbl_unum, plus, minus,
signum, gt, lt, ge, le, numeq, logand, logior,
logxor, logxor_old, bit, bitset, tofloat, toint,
width, c_num, c_fixnum): Extract floating-point value
using c_f accessor. Handle CHR type separately from NUM
because the storage representation is no longer identical;
CHR values have a two bit tag over bits where NUM has
ordinary value bits. NUM is tagged at the NaN level with
the upper 14 bits being 0xFFFC. The remaining 50 bits
are the value.
(flo): Construct unboxed float under NaN boxing by taking
image of double as a 64 bit value, and adding the
delta offset, then casting to the val pointer type.
(c_flo): Separate implementation for NaN boxing.
(integerp, numberp): Use tag_ex.

* buf.c (str_buf, buf_int): Separate CHR and NUM cases,
like in numerous arith.c functions.

* chksum.c (sha256_hash, md5_hash): Use c_ch accessor for
CHR value.

* hash.c (equal_hash, eql_hash): Handle CHR separately. Use
c_f accessor for floating-point value.
(eq_hash): Use tag_ex and handle TAG_FLNUM value under NaN
boxing. Handle CHR separately from NUM.

* ffi.c (ffi_float_put, ffi_double_put, carray_uint, carray_int):
Handle CHR and NUM separately.

* stream.c (formatv): Use c_f accessor.

* configure: disable automatic selection of NaN boxing
on 64 bit platforms, for now.
Add test whether -Wno-strict-aliasing is supported by
the compiler, performed only if NaN boxing is enabled.
We need to disable this warning because it goes off on
the code that reinterprets an integer as a double and
vice versa.

1 files changed, 20 insertions, 1 deletions

diff --git a/configure b/configure
index 671fcfe4..d1fbe948 100755
--- a/configure
+++ b/configure
@@ -1586,7 +1586,8 @@ fi
 if ! [ $nan_boxing_given ] ; then
   printf "Checking whether to use NaN boxing ... "
 
-  if [ $SIZEOF_PTR -eq 8 ] ; then
+  # too experimental: disabled
+  if false && [ $SIZEOF_PTR -eq 8 ] ; then
     nan_boxing=y
     printf "yes\n"
   else
@@ -1601,6 +1602,24 @@ if [ -n "$nan_boxing" ] ; then
   printf "#define CONFIG_NAN_BOXING 1\n" >> config.h
 fi
 
+if [ -n "$nan_boxing" ] ; then
+  printf "Checking how to disable strict aliasing warnings ... "
+
+  cat > conftest.c <<!
+int main(void)
+{
+  return 0;
+}
+!
+
+  if conftest EXTRA_FLAGS=-Wno-strict-aliasing ; then
+    printf -- "-Wno-strict-aliasing\n"
+    diag_flags="$diag_flags -Wno-strict-aliasing"
+  else
+    printf "unknown\n"
+  fi
+fi
+
 printf "Checking for intmax_t  ... "
 cat > conftest.c <<!
 #include <inttypes.h>