From: "Paul A. Clarke" <p...@us.ibm.com>
helper_todouble() was not properly converting any denormalized 32 bit
float to 64 bit double.
Fix-suggested-by: Richard Henderson <richard.hender...@linaro.org>
Signed-off-by: Paul A. Clarke <p...@us.ibm.com>
v2:
- Splitting patch "ppc: Three floating point fixes"; this is just one part.
- Original suggested "fix" was likely flawed. v2 is rewritten by
Richard Henderson (Thanks, Richard!); I reformatted the comments in a
couple of places, compiled, and tested.
Message-Id: <1566250936-14538-1-git-send-email...@us.ibm.com>
Signed-off-by: David Gibson <da...@gibson.dropbear.id.au>
---
target/ppc/fpu_helper.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/target/ppc/fpu_helper.c b/target/ppc/fpu_helper.c
index 52bcda27a6..07bc9051b0 100644
--- a/target/ppc/fpu_helper.c
+++ b/target/ppc/fpu_helper.c
@@ -73,11 +73,20 @@ uint64_t helper_todouble(uint32_t arg)
/* Zero or Denormalized operand. */
ret = (uint64_t)extract32(arg, 31, 1) << 63;
if (unlikely(abs_arg != 0)) {
- /* Denormalized operand. */
- int shift = clz32(abs_arg) - 9;
- int exp = -126 - shift + 1023;
+ /*
+ * Denormalized operand.
+ * Shift fraction so that the msb is in the implicit bit position.
+ * Thus, shift is in the range [1:23].
+ */
+ int shift = clz32(abs_arg) - 8;
+ /*
+ * The first 3 terms compute the float64 exponent. We then bias
+ * this result by -1 so that we can swallow the implicit bit below.
+ */
+ int exp = -126 - shift + 1023 - 1;
+
ret |= (uint64_t)exp << 52;
- ret |= abs_arg << (shift + 29);
+ ret += (uint64_t)abs_arg << (52 - 23 + shift);
}
}
return ret;
--
2.21.0
