[flang-commits] [flang] [flang][runtime] Fix NEAREST() when exponent decreases (PR #75368)

[Peter Klausler via flang-commits]

https://github.com/klausler created https://github.com/llvm/llvm-project/pull/75368

When the result of NEAREST() has an exponent less than that of the argument (e.g., NEAREST(1.,-1.) and NEAREST(-1.,1.)), the result was wrong, because the increment value uses the result of SPACING() in terms of the argument.  Fix by just calling into the C runtime routine std::nextafter().

>From 5a4a7896937ce414af7eb53b5800aa3e107122d6 Mon Sep 17 00:00:00 2001
From: Peter Klausler <pklausler at nvidia.com>
Date: Wed, 13 Dec 2023 11:16:04 -0800
Subject: [PATCH] [flang][runtime] Fix NEAREST() when exponent decreases

When the result of NEAREST() has an exponent less than that of
the argument (e.g., NEAREST(1.,-1.) and NEAREST(-1.,1.)), the
result was wrong, because the increment value uses the result of
SPACING() in terms of the argument.  Fix by just calling into
the C runtime routine std::nextafter().
---
 flang/runtime/numeric.cpp           | 8 +++-----
 flang/unittests/Runtime/Numeric.cpp | 2 +-
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/flang/runtime/numeric.cpp b/flang/runtime/numeric.cpp
index cd54e6b54a2e53..bb711d3b76eaad 100644
--- a/flang/runtime/numeric.cpp
+++ b/flang/runtime/numeric.cpp
@@ -247,12 +247,10 @@ template <int PREC, typename T> inline T Spacing(T x) {
 
 // NEAREST (16.9.139)
 template <int PREC, typename T> inline T Nearest(T x, bool positive) {
-  auto spacing{Spacing<PREC>(x)};
-  if (x == 0) {
-    auto least{std::numeric_limits<T>::denorm_min()};
-    return positive ? least : -least;
+  if (positive) {
+    return std::nextafter(x, std::numeric_limits<T>::infinity());
   } else {
-    return positive ? x + spacing : x - spacing;
+    return std::nextafter(x, -std::numeric_limits<T>::infinity());
   }
 }
 
diff --git a/flang/unittests/Runtime/Numeric.cpp b/flang/unittests/Runtime/Numeric.cpp
index 5afed750c0b183..43263d1ac42315 100644
--- a/flang/unittests/Runtime/Numeric.cpp
+++ b/flang/unittests/Runtime/Numeric.cpp
@@ -86,7 +86,7 @@ TEST(Numeric, Nearest) {
   EXPECT_EQ(RTNAME(Nearest8)(Real<8>{1.0}, true),
       Real<8>{1.0} + std::ldexp(Real<8>{1.0}, -52));
   EXPECT_EQ(RTNAME(Nearest8)(Real<8>{1.0}, false),
-      Real<8>{1.0} - std::ldexp(Real<8>{1.0}, -52));
+      Real<8>{1.0} - 0.5 * std::ldexp(Real<8>{1.0}, -52));
 }
 
 TEST(Numeric, Nint) {