From 68efe6356551237e967c23c1bf5fd01dc4a2fc90 Mon Sep 17 00:00:00 2001
From: Mike Kashkarov <fruitclover@gmail.com>
Date: Tue, 5 Apr 2022 17:09:54 +0300
Subject: [PATCH] [flang] Fix ICE for sqrt(0.0) evaluation

    During real range reduction to [0.5, 4) with

           SQRT(2**(2a) * x) = SQRT(2**(2a)) * SQRT(x) = 2**a * SQRT(x)

    we fall into inf. recursion if IsZero() == true.

    Explicitly handle SQRT(0.0) instead of additional checks during folding. Also
    add helpers for +0.0/-0.0 generation to clean up a bit.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D123131
---
 flang/include/flang/Evaluate/real.h | 4 ++++
 flang/lib/Evaluate/real.cpp         | 8 +++++---
 flang/test/Evaluate/folding28.f90   | 5 +++++
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/flang/include/flang/Evaluate/real.h b/flang/include/flang/Evaluate/real.h
index 14f26dfb52ff..2f4f7b2c4775 100644
--- a/flang/include/flang/Evaluate/real.h
+++ b/flang/include/flang/Evaluate/real.h
@@ -196,6 +196,10 @@ public:
                 .IBSET(significandBits - 2)};
   }
 
+  static constexpr Real PositiveZero() { return Real{}; }
+
+  static constexpr Real NegativeZero() { return {Word{}.MASKL(1)}; }
+
   static constexpr Real Infinity(bool negative) {
     Word infinity{maxExponent};
     infinity = infinity.SHIFTL(significandBits);
diff --git a/flang/lib/Evaluate/real.cpp b/flang/lib/Evaluate/real.cpp
index d273e76d70e1..eb6bd148e341 100644
--- a/flang/lib/Evaluate/real.cpp
+++ b/flang/lib/Evaluate/real.cpp
@@ -98,7 +98,7 @@ ValueWithRealFlags<Real<W, P>> Real<W, P>::Add(
     if (order == Ordering::Equal) {
       // x + (-x) -> +0.0 unless rounding is directed downwards
       if (rounding.mode == common::RoundingMode::Down) {
-        result.value.word_ = result.value.word_.IBSET(bits - 1); // -0.0
+        result.value = NegativeZero();
       }
       return result;
     }
@@ -221,7 +221,7 @@ ValueWithRealFlags<Real<W, P>> Real<W, P>::Divide(
       }
     } else if (IsZero() || y.IsInfinite()) { // 0/x, x/Inf -> 0
       if (isNegative) {
-        result.value.word_ = result.value.word_.IBSET(bits - 1);
+        result.value = NegativeZero();
       }
     } else {
       // dividend and divisor are both finite and nonzero numbers
@@ -272,13 +272,15 @@ ValueWithRealFlags<Real<W, P>> Real<W, P>::SQRT(Rounding rounding) const {
   } else if (IsNegative()) {
     if (IsZero()) {
       // SQRT(-0) == -0 in IEEE-754.
-      result.value.word_ = result.value.word_.IBSET(bits - 1);
+      result.value = NegativeZero();
     } else {
       result.value = NotANumber();
     }
   } else if (IsInfinite()) {
     // SQRT(+Inf) == +Inf
     result.value = Infinity(false);
+  } else if (IsZero()) {
+    result.value = PositiveZero();
   } else {
     int expo{UnbiasedExponent()};
     if (expo < -1 || expo > 1) {
diff --git a/flang/test/Evaluate/folding28.f90 b/flang/test/Evaluate/folding28.f90
index 95db37abe832..004c661692fe 100644
--- a/flang/test/Evaluate/folding28.f90
+++ b/flang/test/Evaluate/folding28.f90
@@ -44,4 +44,9 @@ module m
   logical, parameter :: test_before_1 = sqrt_before_1 == before_1
   real(4), parameter :: sq_sqrt_before_1 = sqrt_before_1 * sqrt_before_1
   logical, parameter :: test_sq_before_1 = sq_sqrt_before_1 < before_1
+  ! ICE at 0.0
+  real(4), parameter :: sqrt_zero_4 = sqrt(0.0)
+  logical, parameter :: test_sqrt_zero_4 = sqrt_zero_4 == 0.0
+  real(8), parameter :: sqrt_zero_8 = sqrt(0.0)
+  logical, parameter :: test_sqrt_zero_8 = sqrt_zero_8 == 0.0
 end module