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Subject[PATCH] RAS/AMD/ATL: Replace bitwise_xor_bits() with hweight16()
Date
Doing hweight16 and checking whether the lsb is set is functionally
equivalent to what bitwise_xor_bits() does. In addition it results in
better generated code as before gcc would inline the function 4 times.
With hweight, the resulting code boils down to 2 instructions - popcnt
and andl as all cpus we care about has popcnt. No functional changes.

An alternative would have been to use the __builtin_parity() function provided
by both Clang/GCC, however under some circumstances the compiler can choose not
to inline it but generate a library call which is unsupported in the kernel.

Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
---
drivers/ras/amd/atl/umc.c | 21 +++++----------------
1 file changed, 5 insertions(+), 16 deletions(-)

diff --git a/drivers/ras/amd/atl/umc.c b/drivers/ras/amd/atl/umc.c
index 6e072b7667e9..8d1363237eee 100644
--- a/drivers/ras/amd/atl/umc.c
+++ b/drivers/ras/amd/atl/umc.c
@@ -49,17 +49,6 @@ static u8 get_coh_st_inst_id_mi300(struct atl_err *err)
return i;
}

-/* XOR the bits in @val. */
-static u16 bitwise_xor_bits(u16 val)
-{
- u16 tmp = 0;
- u8 i;
-
- for (i = 0; i < 16; i++)
- tmp ^= (val >> i) & 0x1;
-
- return tmp;
-}

struct xor_bits {
bool xor_enable;
@@ -250,17 +239,17 @@ static unsigned long convert_dram_to_norm_addr_mi300(unsigned long addr)
if (!addr_hash.bank[i].xor_enable)
continue;

- temp = bitwise_xor_bits(col & addr_hash.bank[i].col_xor);
- temp ^= bitwise_xor_bits(row & addr_hash.bank[i].row_xor);
+ temp = hweight16(col & addr_hash.bank[i].col_xor) & 1;
+ temp ^= hweight16(row & addr_hash.bank[i].row_xor) & 1;
bank ^= temp << i;
}

/* Calculate hash for PC bit. */
if (addr_hash.pc.xor_enable) {
- temp = bitwise_xor_bits(col & addr_hash.pc.col_xor);
- temp ^= bitwise_xor_bits(row & addr_hash.pc.row_xor);
+ temp = hweight16(col & addr_hash.pc.col_xor) & 1;
+ temp ^= hweight16(row & addr_hash.pc.row_xor) & 1;
/* Bits SID[1:0] act as Bank[5:4] for PC hash, so apply them here. */
- temp ^= bitwise_xor_bits((bank | sid << NUM_BANK_BITS) & addr_hash.bank_xor);
+ temp ^= hweight16((bank | sid << NUM_BANK_BITS) & addr_hash.bank_xor) & 1;
pc ^= temp;
}

--
2.52.0

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