target/i386: emulate: microoptimize and explain ADD_COUT_VEC/SUB_COUT_VEC

The logic is the same, but the majority(NOT a, b, c) is brought out
to a separate macro and implemented without NOT operations.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/target/i386/hvf/x86_flags.c b/target/i386/hvf/x86_flags.c
index fedc70a1b803a8fe17054f6d76874848f7a48c81..60ab4f01a203a2182fd0cc81405ff26b590957ed 100644 (file)
--- a/target/i386/hvf/x86_flags.c
+++ b/target/i386/hvf/x86_flags.c
@@ -45,14 +45,30 @@
 #define LF_MASK_CF     (0x01 << LF_BIT_CF)
 #define LF_MASK_PO     (0x01 << LF_BIT_PO)
 
+/* majority(NOT a, b, c) = (a ^ b) ? b : c */
+#define MAJ_INV1(a, b, c)  ((((a) ^ (b)) & ((b) ^ (c))) ^ (c))
+
+/*
+ * ADD_COUT_VEC(x, y) = majority((x + y) ^ x ^ y, x, y)
+ *
+ * If two corresponding bits in x and y are the same, that's the carry
+ * independent of the value (x+y)^x^y.  Hence x^y can be replaced with
+ * 1 in (x+y)^x^y, resulting in majority(NOT (x+y), x, y)
+ */
 #define ADD_COUT_VEC(op1, op2, result) \
-   (((op1) & (op2)) | (((op1) | (op2)) & (~(result))))
+   MAJ_INV1(result, op1, op2)
 
+/*
+ * SUB_COUT_VEC(x, y) = NOT majority(x, NOT y, (x - y) ^ x ^ NOT y)
+ *                    = majority(NOT x, y, (x - y) ^ x ^ y)
+ *
+ * Note that the carry out is actually a borrow, i.e. it is inverted.
+ * If two corresponding bits in x and y are different, the value of the
+ * bit in (x-y)^x^y likewise does not matter.  Hence, x^y can be replaced
+ * with 0 in (x-y)^x^y, resulting in majority(NOT x, y, x-y)
+ */
 #define SUB_COUT_VEC(op1, op2, result) \
-   (((~(op1)) & (op2)) | (((~(op1)) ^ (op2)) & (result)))
-
-#define GET_ADD_OVERFLOW(op1, op2, result, mask) \
-   ((((op1) ^ (result)) & ((op2) ^ (result))) & (mask))
+   MAJ_INV1(op1, op2, result)
 
 /* ******************* */
 /* OSZAPC */