Hi Marc,
On 13/12/17 14:32, Marc Zyngier wrote:
> On 12/12/17 18:32, James Morse wrote:
>> On 11/12/17 14:49, Marc Zyngier wrote:
>>> We lack a way to encode operations such as AND, ORR, EOR that take
>>> an immediate value. Doing so is quite involved, and is all about
>>> reverse engineering the decoding algorithm described in the
>>> pseudocode function DecodeBitMasks().
>>> diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
>>> index 7e432662d454..326b17016485 100644
>>> --- a/arch/arm64/kernel/insn.c
>>> +++ b/arch/arm64/kernel/insn.c
>>
>>> +static u32 aarch64_encode_immediate(u64 imm,
>>> + enum aarch64_insn_variant variant,
>>> + u32 insn)
>>> +{
>>> + unsigned int immr, imms, n, ones, ror, esz, tmp;
>>> + u64 mask;
>>
>> [...]
>>
>>> + /* Compute the rotation */
>>> + if (range_of_ones(imm)) {
>>> + /*
>>> + * Pattern: 0..01..10..0
>>> + *
>>> + * Compute how many rotate we need to align it right
>>> + */
>>> + ror = ffs(imm) - 1;
>>
>> (how come range_of_ones() uses __ffs64() on the same value?)
>
> News flash: range_of_ones is completely buggy. It will fail on the
> trivial value 1 (__ffs64(1) = 0; 0 - 1 = -1; val >> -1 is... ermmmm).
> I definitely got mixed up between the two.
They do different things!? Aaaaaahhhh....
[ ...]
>> Unless I've gone wrong, I think the 'Trim imm to the element size' code needs to
>> move up into the esz-reducing loop so it doesn't happen for a 64bit immediate.
> Yup. I've stashed the following patch:
>
> diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
> index b8fb2d89b3a6..e58be1c57f18 100644
> --- a/arch/arm64/kernel/insn.c
> +++ b/arch/arm64/kernel/insn.c
> @@ -1503,8 +1503,7 @@ pstate_check_t * const aarch32_opcode_cond_checks[16] = {
> static bool range_of_ones(u64 val)
> {
> /* Doesn't handle full ones or full zeroes */
> - int x = __ffs64(val) - 1;
> - u64 sval = val >> x;
> + u64 sval = val >> __ffs64(val);
>
> /* One of Sean Eron Anderson's bithack tricks */
> return ((sval + 1) & (sval)) == 0;
> @@ -1515,7 +1514,7 @@ static u32 aarch64_encode_immediate(u64 imm,
> u32 insn)
> {
> unsigned int immr, imms, n, ones, ror, esz, tmp;
> - u64 mask;
> + u64 mask = ~0UL;
>
> /* Can't encode full zeroes or full ones */
> if (!imm || !~imm)
> @@ -1543,8 +1542,12 @@ static u32 aarch64_encode_immediate(u64 imm,
> for (tmp = esz; tmp > 2; tmp /= 2) {
> u64 emask = BIT(tmp / 2) - 1;
>
> - if ((imm & emask) != ((imm >> (tmp / 2)) & emask))
> + if ((imm & emask) != ((imm >> (tmp / 2)) & emask)) {
> + /* Trim imm to the element size */
> + mask = BIT(esz - 1) - 1;
> + imm &= mask;
Won't this still lose the top bit? It generates 0x7fffffff for esz=32, and for
esz=32 we run through here when the two 16bit values are different.
This still runs for a 64bit immediate. The 0xf80000000fffffff example compares
0xf8000000 with 0fffffff then breaks here on the first iteration of this loop.
With this change it still attempts to generate a 64bit mask.
I was thinking of something like [0]. That only runs when we know the two
tmp:halves match, it just keeps the bottom tmp:half for the next run and never
runs for a 64bit immediate.
> break;
> + }
>
> esz = tmp;
> }
> @@ -1552,10 +1555,6 @@ static u32 aarch64_encode_immediate(u64 imm,
> /* N is only set if we're encoding a 64bit value */
> n = esz == 64;
>
> - /* Trim imm to the element size */
> - mask = BIT(esz - 1) - 1;
> - imm &= mask;
> -
> /* That's how many ones we need to encode */
> ones = hweight64(imm);
>
> I really need to run this against gas in order to make sure
> I get the same parameters for all the possible values.
Sounds good,
Thanks,
James
[0] Not even built:
diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
index 12d3ec2154c2..d9fbdea7b18d 100644
--- a/arch/arm64/kernel/insn.c
+++ b/arch/arm64/kernel/insn.c
@@ -1529,15 +1529,15 @@ static u32 aarch64_encode_immediate(u64 imm,
break;
esz = tmp;
+
+ /* Trim imm to the element size */
+ mask = BIT(esz) - 1;
+ imm &= mask;
}
/* N is only set if we're encoding a 64bit value */
n = esz == 64;
- /* Trim imm to the element size */
- mask = BIT(esz - 1) - 1;
- imm &= mask;
-
/* That's how many ones we need to encode */
ones = hweight64(imm);
