From f6e0ec5696ec5f52baed71fe23f978bcef80d458 Mon Sep 17 00:00:00 2001 From: Jakub Jelinek Date: Wed, 6 Sep 2023 17:42:37 +0200 Subject: [PATCH] libgcc _BitInt helper documentation [PR102989] On Mon, Aug 21, 2023 at 05:32:04PM +0000, Joseph Myers wrote: > I think the libgcc functions (i.e. those exported by libgcc, to which > references are generated by the compiler) need documenting in libgcc.texi. > Internal functions or macros in the libgcc patch need appropriate comments > specifying their semantics; especially FP_TO_BITINT and FP_FROM_BITINT > which have a lot of arguments and no comments saying what the semantics of > the macros and their arguments are supposed to me. Here is an incremental patch which does that. 2023-09-06 Jakub Jelinek PR c/102989 gcc/ * doc/libgcc.texi (Bit-precise integer arithmetic functions): Document general rules for _BitInt support library functions and document __mulbitint3 and __divmodbitint4. (Conversion functions): Document __fix{s,d,x,t}fbitint, __floatbitint{s,d,x,t,h,b}f, __bid_fix{s,d,t}dbitint and __bid_floatbitint{s,d,t}d. libgcc/ * libgcc2.c (bitint_negate): Add function comment. * soft-fp/bitint.h (bitint_negate): Add function comment. (FP_TO_BITINT, FP_FROM_BITINT): Add comment explaining the macros. --- gcc/doc/libgcc.texi | 83 +++++++++++++++++++++++++++++++++++++++++ libgcc/libgcc2.c | 2 + libgcc/soft-fp/bitint.h | 26 +++++++++++++ 3 files changed, 111 insertions(+) diff --git a/gcc/doc/libgcc.texi b/gcc/doc/libgcc.texi index 73aa803f97b3..08219bfc052e 100644 --- a/gcc/doc/libgcc.texi +++ b/gcc/doc/libgcc.texi @@ -218,6 +218,51 @@ These functions return the number of bits set in @var{a}. These functions return the @var{a} byteswapped. @end deftypefn +@subsection Bit-precise integer arithmetic functions + +@code{_BitInt(@var{n})} library functions operate on arrays of limbs, where +each limb has @code{__LIBGCC_BITINT_LIMB_WIDTH__} bits and the limbs are +ordered according to @code{__LIBGCC_BITINT_ORDER__} ordering. The most +significant limb if @var{n} is not divisible by +@code{__LIBGCC_BITINT_LIMB_WIDTH__} contains padding bits which should be +ignored on read (sign or zero extended), but extended on write. For the +library functions, all bit-precise integers regardless of @var{n} are +represented like that, even when the target ABI says that for some small +@var{n} they should be represented differently in memory. A pointer +to the array of limbs argument is always accompanied with a bit size +argument. If that argument is positive, it is number of bits and the +number is assumed to be zero-extended to infinite precision, if that +argument is negative, it is negated number of bits above which all bits +are assumed to be sign-extended to infinite precision. These number of bits +arguments don't need to match actual @var{n} for the operation used in the +source, they could be lowered because of sign or zero extensions on the +input or because value-range optimization figures value will need certain +lower number of bits. For big-endian ordering of limbs, when lowering +the bit size argument the pointer argument needs to be adjusted as well. +Negative bit size argument should be always smaller or equal to @code{-2}, +because @code{signed _BitInt(1)} is not valid. +For output arguments, either the corresponding bit size argument should +be always positive (for multiplication and division), or is negative when +the output of conversion from floating-point value is signed and positive +when unsigned. The arrays of limbs output arguments point to should not +overlap any inputs, while input arrays of limbs can overlap. +@code{UBILtype} below stands for unsigned integer type with +@code{__LIBGCC_BITINT_LIMB_WIDTH__} bit precision. + +@deftypefn {Runtime Function} void __mulbitint3 (@code{UBILtype} *@var{ret}, int32_t @var{retprec}, const @code{UBILtype} *u, int32_t @var{uprec}, const @code{UBILtype} *v, int32_t @var{vprec}) +This function multiplies bit-precise integer operands @var{u} and @var{v} and stores +result into @var{retprec} precision bit-precise integer result @var{ret}. +@end deftypefn + +@deftypefn {Runtime Function} void __divmodbitint4 (@code{UBILtype} *@var{q}, int32_t @var{qprec}, @code{UBILtype} *@var{r}, int32_t @var{rprec}, const @code{UBILtype} *u, int32_t @var{uprec}, const @code{UBILtype} *v, int32_t @var{vprec}) +This function divides bit-precise integer operands @var{u} and @var{v} and stores +quotient into @var{qprec} precision bit-precise integer result @var{q} +(unless @var{q} is @code{NULL} and @var{qprec} is 0, in that case quotient +is not stored anywhere) and remainder into @var{rprec} precision bit-precise +integer result @var{r} (similarly, unless @var{r} is @code{NULL} and @var{rprec} +is 0). +@end deftypefn + @node Soft float library routines @section Routines for floating point emulation @cindex soft float library @@ -384,6 +429,27 @@ These functions convert @var{i}, an unsigned long, to floating point. These functions convert @var{i}, an unsigned long long, to floating point. @end deftypefn +@deftypefn {Runtime Function} void __fixsfbitint (@code{UBILtype} *@var{r}, int32_t @var{rprec}, float @var{a}) +@deftypefnx {Runtime Function} void __fixdfbitint (@code{UBILtype} *@var{r}, int32_t @var{rprec}, double @var{a}) +@deftypefnx {Runtime Function} void __fixxfbitint (@code{UBILtype} *@var{r}, int32_t @var{rprec}, __float80 @var{a}) +@deftypefnx {Runtime Function} void __fixtfbitint (@code{UBILtype} *@var{r}, int32_t @var{rprec}, _Float128 @var{a}) +These functions convert @var{a} to bit-precise integer @var{r}, rounding toward zero. +If @var{rprec} is positive, it converts to unsigned bit-precise integer and +negative values all become zero, if @var{rprec} is negative, it converts +to signed bit-precise integer. +@end deftypefn + +@deftypefn {Runtime Function} float __floatbitintsf (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +@deftypefnx {Runtime Function} double __floatbitintdf (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +@deftypefnx {Runtime Function} __float80 __floatbitintxf (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +@deftypefnx {Runtime Function} _Float128 __floatbitinttf (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +@deftypefnx {Runtime Function} _Float16 __floatbitinthf (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +@deftypefnx {Runtime Function} __bf16 __floatbitintbf (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +These functions convert bit-precise integer @var{i} to floating point. If +@var{iprec} is positive, it is conversion from unsigned bit-precise integer, +otherwise from signed bit-precise integer. +@end deftypefn + @subsection Comparison functions There are two sets of basic comparison functions. @@ -707,6 +773,23 @@ These functions convert @var{i}, an unsigned integer, to decimal floating point. These functions convert @var{i}, an unsigned long, to decimal floating point. @end deftypefn +@deftypefn {Runtime Function} void __bid_fixsdbitint (@code{UBILtype} *@var{r}, int32_t @var{rprec}, _Decimal32 @var{a}) +@deftypefnx {Runtime Function} void __bid_fixddbitint (@code{UBILtype} *@var{r}, int32_t @var{rprec}, _Decimal64 @var{a}) +@deftypefnx {Runtime Function} void __bid_fixtdbitint (@code{UBILtype} *@var{r}, int32_t @var{rprec}, _Decimal128 @var{a}) +These functions convert @var{a} to bit-precise integer @var{r}, rounding toward zero. +If @var{rprec} is positive, it converts to unsigned bit-precise integer and +negative values all become zero, if @var{rprec} is negative, it converts +to signed bit-precise integer. So far implemented for BID format only. +@end deftypefn + +@deftypefn {Runtime Function} _Decimal32 __bid_floatbitintsd (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +@deftypefnx {Runtime Function} _Decimal64 __bid_floatbitintdd (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +@deftypefnx {Runtime Function} _Decimal128 __bid_floatbitinttd (@code{UBILtype} *@var{i}, int32_t @var{iprec}) +These functions convert bit-precise integer @var{i} to decimal floating point. If +@var{iprec} is positive, it is conversion from unsigned bit-precise integer, +otherwise from signed bit-precise integer. So far implemented for BID format only. +@end deftypefn + @subsection Comparison functions @deftypefn {Runtime Function} int __dpd_unordsd2 (_Decimal32 @var{a}, _Decimal32 @var{b}) diff --git a/libgcc/libgcc2.c b/libgcc/libgcc2.c index d217e339c21b..8b42210524f7 100644 --- a/libgcc/libgcc2.c +++ b/libgcc/libgcc2.c @@ -1640,6 +1640,8 @@ __mulbitint3 (UWtype *ret, SItype retprec, #endif #ifdef L_divmodbitint4 +/* D = -S. */ + static void bitint_negate (UWtype *d, const UWtype *s, SItype n) { diff --git a/libgcc/soft-fp/bitint.h b/libgcc/soft-fp/bitint.h index 5e8b76407f43..20cd41ba1156 100644 --- a/libgcc/soft-fp/bitint.h +++ b/libgcc/soft-fp/bitint.h @@ -160,6 +160,9 @@ bitint_reduce_prec (const UBILtype **p, SItype prec) # define BITINT_END(be, le) (le) #endif +/* Negate N limbs from S into D. D and S should point to + the least significant limb. */ + static inline __attribute__((__always_inline__)) void bitint_negate (UBILtype *d, const UBILtype *s, SItype n) { @@ -175,6 +178,19 @@ bitint_negate (UBILtype *d, const UBILtype *s, SItype n) while (--n); } +/* Common final part of __fix?fbitint conversion functions. + The A floating point value should have been converted using + soft-fp macros into RV, U##DI##type DI##_BITS precise normal + integral type and SHIFT, how many bits should that value be + shifted to the left. R is pointer to limbs array passed to the + function, RN number of limbs in it, ARPREC absolute value of + RPREC argument passed to it, RSIZE number of significant bits in RV. + RSIGNED is non-zero if the result is signed bit-precise integer, + otherwise zero. If OVF is true, instead of storing RV shifted left + by SHIFT bits and zero or sign extended store minimum or maximum + of the signed or unsigned bit-precise integer type or zero depending on if + RV contains the minimum or maximum signed or unsigned value or zero. */ + #define FP_TO_BITINT(r, rn, arprec, shift, rv, rsize, rsigned, ovf, DI) \ if (ovf) \ { \ @@ -232,6 +248,16 @@ bitint_negate (UBILtype *d, const UBILtype *s, SItype n) * sizeof (UBILtype)); \ } +/* Common initial part of __floatbitint?f conversion functions. + I and IPREC are arguments passed to those functions, convert that + into a pair of DI##type IV integer and SHIFT, such that converting + IV to floating point and multiplicating that by pow (2, SHIFT) + gives the expected result. IV size needs to be chosen such that + it is larger than number of bits in floating-point mantissa and + contains there even at least a two bits below the mantissa for + rounding purposes. If any of the SHIFT bits shifted out is non-zero, + the least significant bit should be non-zero. */ + #define FP_FROM_BITINT(i, iprec, iv, shift, DI) \ do \ { \ -- 2.43.5