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[PULL,2/4] softfloat: fix floatx80 pseudo-denormal addition / subtraction

Message ID 20200515184722.31182-3-richard.henderson@linaro.org
State Accepted
Commit 41602807766e253ccb6fb761f3ff12767f786e2c
Headers show
Series softfloat patch queue | expand

Commit Message

Richard Henderson May 15, 2020, 6:47 p.m. UTC 
From: Joseph Myers <joseph@codesourcery.com>


The softfloat function addFloatx80Sigs, used for addition of values
with the same sign and subtraction of values with opposite sign, fails
to handle the case where the two values both have biased exponent zero
and there is a carry resulting from adding the significands, which can
occur if one or both values are pseudo-denormals (biased exponent
zero, explicit integer bit 1).  Add a check for that case, so making
the results match those seen on x86 hardware for pseudo-denormals.

Signed-off-by: Joseph Myers <joseph@codesourcery.com>

Message-Id: <alpine.DEB.2.21.2005042337570.22972@digraph.polyomino.org.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

---
 fpu/softfloat.c                            |  6 ++++++
 tests/tcg/i386/test-i386-pseudo-denormal.c | 24 ++++++++++++++++++++++
 2 files changed, 30 insertions(+)
 create mode 100644 tests/tcg/i386/test-i386-pseudo-denormal.c

-- 
2.20.1
diff mbox series

Patch

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index ac116c70b8..6094d267b5 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -5866,6 +5866,12 @@  static floatx80 addFloatx80Sigs(floatx80 a, floatx80 b, flag zSign,
         zSig1 = 0;
         zSig0 = aSig + bSig;
         if ( aExp == 0 ) {
+            if ((aSig | bSig) & UINT64_C(0x8000000000000000) && zSig0 < aSig) {
+                /* At least one of the values is a pseudo-denormal,
+                 * and there is a carry out of the result.  */
+                zExp = 1;
+                goto shiftRight1;
+            }
             if (zSig0 == 0) {
                 return packFloatx80(zSign, 0, 0);
             }
diff --git a/tests/tcg/i386/test-i386-pseudo-denormal.c b/tests/tcg/i386/test-i386-pseudo-denormal.c
new file mode 100644
index 0000000000..cfa2a500b0
--- /dev/null
+++ b/tests/tcg/i386/test-i386-pseudo-denormal.c
@@ -0,0 +1,24 @@ 
+/* Test pseudo-denormal operations.  */
+
+#include <stdint.h>
+#include <stdio.h>
+
+union u {
+    struct { uint64_t sig; uint16_t sign_exp; } s;
+    long double ld;
+};
+
+volatile union u ld_pseudo_m16382 = { .s = { UINT64_C(1) << 63, 0 } };
+
+volatile long double ld_res;
+
+int main(void)
+{
+    int ret = 0;
+    ld_res = ld_pseudo_m16382.ld + ld_pseudo_m16382.ld;
+    if (ld_res != 0x1p-16381L) {
+        printf("FAIL: pseudo-denormal add\n");
+        ret = 1;
+    }
+    return ret;
+}