Hello Stefan, Segher,
- I still need to try Seghers suggestion. Let me first answer Stephan.
Take following piece of C-code:
int check_test(int a, int b, int c, int d)
{
_Bool check = (d == c) & (a > b) ;
if(check)
{
b = a ;
}
return(b) ;
}
- What I want, is that the compiler does following:
1. _Bool var1 = (d == c)
2. _Bool var2 = (a > b)
3. _Bool check = var1 & var2
if(check) then conditional branch ...
- Now, the core has a condition code register,
does 1. and 2. with a compare instruction, 3 with a crand.
When I put the var1 and var2 in the condition register, this works !.
Well, there are still some bugs, but this is what it does :
// The machine passes arguments in registers, a:R20, b: R21 and so on.
comparison internal signed %cf9 %r23 %r22 // (it writes 3
flags. CF9 is less than, CF10 is greater, CF11 is equal
comparison internal signed %cf6 %r20 %r21 // writes CF6, CF7
and CF8
crand %cf9 %cf7 %cf11
novec_mybranch %cf9 1 .L2
addi %r20, %r21, 0
.L2:
ret
// return in register 20.
Now, Stephan, how do you intend to do the boolean and, if you put the
boolean variables in the SI (32-bit) register file of the machine ?. I mean,
then you need to move the content of the CC reg to an R-reg, AND it, and
move
back from R-reg to CC-reg. The other solution is 2 branches ?.
Best Regards,
Henri.
On 01/30/2020 11:57 AM, Stefan Franke wrote:
-----Ursprüngliche Nachricht-----
Von: gcc-help-owner@xxxxxxxxxxx <gcc-help-owner@xxxxxxxxxxx> Im
Auftrag von Segher Boessenkool
Gesendet: Donnerstag, 30. Januar 2020 11:48
An: Henri Cloetens <henri.cloetens@xxxxxxxxxx>
Cc: gcc-help@xxxxxxxxxxx
Betreff: Re: Issue with subregs
Hi!
On Thu, Jan 30, 2020 at 11:19:18AM +0100, Henri Cloetens wrote:
- It is indeed true subreg only works on bytes.
- So, I changed the declaration:
a. I declared all 12 condition code bits as BYTES (QI)
That's not going to work. The separate 3-bit fields are still at offsets
0, 3, 6, 9
bit. You cannot use subregs to access them.
b. I split it in 4 CC fields (cfr below), and declared each of these
as SI.
Why not as CCmode? That's what they are, after all.
- Then, the compare addresses the CC field in SI mode, the branch in
QI
mode, and all goes well. I mean, it does not matter if I declare SI
or BI, because
the compare instruction only writes 0 or 1, and the branch
instruction tests only
for 0 or 1.
Or do the compare insns only set single bits? Not (e.g.) all of "less
than",
"equal to", "greater than" at once?
Segher
My approach was: group all bits which may change simultaneously into one CC
reg each
This may result into one or more CC regs .
I got two of these
Create at least a mode for each CC reg for each used combination
I got two modes for the first CC reg: one to reflect eq/ne and one
mode for all other
And one mode for the second CC reg, which covers all conditions
Then I coded the cbranch/cstore expands accordingly
A CC reg is either used in a branch or sets the boolean value 0/1 in a
DI/SI/HI/QI register.
You'll may do more work later, if you want to support insn resulting by the
combine pass, e.g. testing single bits...
cheers
Stefan
