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[avr,applied] PR115419: round double to even LSB on tie breaks

Message ID e1c13f69-5062-4284-95d5-1e657ef3bec7@gjlay.de
State New
Headers show
Series [avr,applied] PR115419: round double to even LSB on tie breaks | expand

Commit Message

Georg-Johann Lay June 14, 2024, 4:36 p.m. UTC 
The convention in round-to-nearest floating-point
rounding is to round to even ("Banker's rounding").

Johann

AVR: target/115419 - Tie breaks are rounded-to-even.

libgcc/config/avr/libf7/
	PR target/115419
	* libf7.c (f7_get_double): Round tie breaks to even LSB.
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Patch

diff --git a/libgcc/config/avr/libf7/libf7.c 
b/libgcc/config/avr/libf7/libf7.c
index 375becb854c..6fae4fc1a2d 100644
--- a/libgcc/config/avr/libf7/libf7.c
+++ b/libgcc/config/avr/libf7/libf7.c
@@ -440,11 +440,21 @@  f7_double_t f7_get_double (const f7_t *aa)

    mant &= 0x00ffffffffffffff;

-  // FIXME: For subnormals, rounding is premature and should be
-  //       done *after* the mantissa has been shifted into place
-  //       (or the round value be shifted left accordingly).
-  // Round.
-  mant += 1u << (F7_MANT_BITS - (1 + DBL_DIG_MANT) - 1);
+  // PR115419: The least significant nibble tells how to round:
+  // Tie breaks are rounded to even (Banker's rounding).
+  uint8_t lsn = mant & 0xff;
+  lsn &= 0xf;
+  // The LSB of the outgoing double is at bit 3.
+  if (lsn & (1 << 3))
+    ++lsn;
+  if (lsn > (1 << 2))
+    {
+      // FIXME: For subnormals, rounding is premature and should be
+      //        done *after* the mantissa has been shifted into place
+      //        (or the round value be shifted left accordingly).
+      // Round.
+      mant += 1u << (F7_MANT_BITS - (1 + DBL_DIG_MANT) - 1);
+    }

    uint8_t dex;
    register uint64_t r18 __asm ("r18") = mant;