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[libc][stdfix] Implement fxdivi functions (rdivi) #154914

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Merged
merged 14 commits into from
Oct 8, 2025
Merged

[libc][stdfix] Implement fxdivi functions (rdivi) #154914

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f54208e
[libc][stdfix] Implement fxdivi functions
bojle Aug 22, 2025
040e833
[libc][stdfix] add a couple more tests and error info for divi
bojle Aug 30, 2025
71d283b
[libc] add a few tests to handles edge cases with FRACT_{MIN/MAX}
bojle Sep 5, 2025
cfe5122
[libc] fix formatting for rdivi.cpp
bojle Sep 5, 2025
c614888
[libc] comments, remove extra iterations, and misc changes
bojle Sep 13, 2025
40cf04a
[libc] formatting
bojle Sep 25, 2025
744463c
[libc] INT_MIN handling
bojle Sep 26, 2025
f5ff751
[libc] formatting
bojle Sep 26, 2025
31bbc8b
[libc] multiply and divide by 2 when INT_MIN
bojle Sep 26, 2025
b631411
[libc] specially handle po2 cases
bojle Oct 5, 2025
73a93c3
[libc] add special handling for po2
bojle Oct 8, 2025
303dc94
[libc] use abs from algorithm.h
bojle Oct 8, 2025
07e1de8
[libc] add doc, fix func name case
bojle Oct 8, 2025
c2b4b99
[libc] TotalBits -> TOTAL_BITS
bojle Oct 8, 2025
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43 changes: 32 additions & 11 deletions libc/src/__support/fixed_point/fx_bits.h
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,8 @@

#include "fx_rep.h"

#include <stdio.h>

#ifdef LIBC_COMPILER_HAS_FIXED_POINT

namespace LIBC_NAMESPACE_DECL {
Expand Down Expand Up @@ -244,8 +246,35 @@ template <typename XType> LIBC_INLINE constexpr XType divi(int n, int d) {
if (LIBC_UNLIKELY(n == 0)) {
return FXRep<XType>::ZERO();
}
bool result_is_negative = ((n < 0) != (d < 0));
auto isPowerOfTwo = [](int n) { return (n > 0) && ((n & (n - 1)) == 0); };
long accum max_val = static_cast<long accum>(FXRep<XType>::MAX());
long accum min_val = static_cast<long accum>(FXRep<XType>::MIN());

if (isPowerOfTwo(cpp::abs(d))) {
int k = cpp::countr_zero<uint32_t>(static_cast<uint32_t>(cpp::abs(d)));
constexpr int F = FXRep<XType>::FRACTION_LEN;
int64_t scaled_n = static_cast<int64_t>(n) << F;
int64_t res64 = scaled_n >> k;
constexpr int TotalBits = sizeof(XType) * 8;
const int64_t max_limit = (1LL << (TotalBits - 1)) - 1;
const int64_t min_limit = -(1LL << (TotalBits - 1));
if (res64 > max_limit) {
return FXRep<XType>::MAX();
} else if (res64 < min_limit) {
return FXRep<XType>::MIN();
}
long accum res_accum =
static_cast<long accum>(res64) / static_cast<long accum>(1 << F);
res_accum = (d < 0) ? static_cast<long accum>(-1) * res_accum : res_accum;
if (res_accum > max_val) {
return FXRep<XType>::MAX();
} else if (res_accum < min_val) {
return FXRep<XType>::MIN();
}
return static_cast<XType>(res_accum);
}

bool result_is_negative = ((n < 0) != (d < 0));
int64_t n64 = static_cast<int64_t>(n);
int64_t d64 = static_cast<int64_t>(d);

Expand Down Expand Up @@ -281,14 +310,10 @@ template <typename XType> LIBC_INLINE constexpr XType divi(int n, int d) {
// to quadratic convergence. So,
// E1 = (0.059)^2 = 0.0034
long accum val = nrstep(d_scaled_val, initial_approx);
auto isPowerOfTwo = [](int n) { return (n > 0) && ((n & (n - 1)) == 0); };
// Division with a power of 2 would generally be expected to be
// exact, we handle this by specially treating po2 cases and having
// extra iterations for them.
if (FXRep<XType>::FRACTION_LEN > 8 || isPowerOfTwo(cpp::abs(d))) {
if constexpr (FXRep<XType>::FRACTION_LEN > 8) {
// E2 = 0.0000121
val = nrstep(d_scaled_val, val);
if (FXRep<XType>::FRACTION_LEN > 16 || isPowerOfTwo(cpp::abs(d))) {
if constexpr (FXRep<XType>::FRACTION_LEN > 16) {
// E3 = 1.468e−10
val = nrstep(d_scaled_val, val);
}
Expand All @@ -299,10 +324,6 @@ template <typename XType> LIBC_INLINE constexpr XType divi(int n, int d) {
res *= static_cast<long accum>(-1);
}

// Check for overflow before returning
long accum max_val = static_cast<long accum>(FXRep<XType>::MAX());
long accum min_val = static_cast<long accum>(FXRep<XType>::MIN());

// Per clause 7.18a.6.1, saturate values on overflow
if (res > max_val) {
return FXRep<XType>::MAX();
Expand Down
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