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

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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>
This commit is contained in:
Paolo Bonzini 2025-03-20 14:55:42 +01:00
parent 3fec86e95c
commit 767149d3d0
1 changed files with 21 additions and 5 deletions
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26
target/i386/hvf/x86_flags.c
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@ -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 */