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feefrac: rework comments around Mul/MulFallback

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This commit is contained in:
Pieter Wuille 2024-07-29 08:03:50 -04:00
parent ebd0daa74a
commit 4b923b602c
1 changed files with 5 additions and 7 deletions
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12
src/util/feefrac.h
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@ -37,23 +37,21 @@
*/ */
struct FeeFrac struct FeeFrac
{ {
/** Fallback version for Mul (see below). /** Helper function for 32*64 signed multiplication, returning an unspecified but totally
* * ordered type. This is a fallback version, separate so it can be tested on platforms where
* Separate to permit testing on platforms where it isn't actually needed. * it isn't actually needed. */
*/
static inline std::pair<int64_t, uint32_t> MulFallback(int64_t a, int32_t b) noexcept static inline std::pair<int64_t, uint32_t> MulFallback(int64_t a, int32_t b) noexcept
{ {
// Otherwise, emulate 96-bit multiplication using two 64-bit multiplies.
int64_t low = int64_t{static_cast<uint32_t>(a)} * b; int64_t low = int64_t{static_cast<uint32_t>(a)} * b;
int64_t high = (a >> 32) * b; int64_t high = (a >> 32) * b;
return {high + (low >> 32), static_cast<uint32_t>(low)}; return {high + (low >> 32), static_cast<uint32_t>(low)};
} }
// Compute a * b, returning an unspecified but totally ordered type.
#ifdef __SIZEOF_INT128__ #ifdef __SIZEOF_INT128__
/** Helper function for 32*64 signed multiplication, returning an unspecified but totally
* ordered type. This is a version relying on __int128. */
static inline __int128 Mul(int64_t a, int32_t b) noexcept static inline __int128 Mul(int64_t a, int32_t b) noexcept
{ {
// If __int128 is available, use 128-bit wide multiply.
return __int128{a} * b; return __int128{a} * b;
} }
#else #else