PowerPC: Missing bdnzflr?

[PeterMatula]

It seems like there is no bdnzflr (PPC_INS_BDNZFLR) instruction in Capstone (next branch). It is possible that its semantics can be expressed using some other instructions -- it is a simplified branch mnemonic instructions, so basic bclr should definitely do the job. But, all the other simplified mnemonics are there, including very similar bdnzflrl (PPC_INS_BDNZFLRL), so it seems like this one should also be present.

[PeterMatula]

According to the output of my Capstone dumper utility program (dumps everything Capstone knows about an instruction) it really is disassembled into PPC_INS_BCLR -- which is by itself ok. But it is harder to work with this instruction than with dedicated missing PPC_INS_BDNZFLR, because the only hint that it decrements and branches using CTR is by registers read/written:

$ capstone-dumper -a ppc -m 32 -e big -b 0x10000510 -t "bdnzflr 4*cr4+lt"
                                                                                                                                                                                                                                                                               
Keystone input : bdnzflr 4*cr4+lt                                                                                                                                                                                                                                              
Keystone output: 4c 10 00 20                                                                                                                                                                                                                                                   

Capstone version: 1024 (major: 4, minor: 0)                                                                                                                                                                                                                                    

#0                                                                                                                                                                                                                                                                             
        General info:                                                                                                                                                                                                                                                          
                id     :  20 (bclr)                                                                                                                                                                                                                                            
                addr   :  10000510                                                                                                                                                                                                                                             
                size   :  4                                                                                                                                                                                                                                                    
                bytes  :  4c 10 00 20                                                                                                                                                                                                                                          
                mnem   :  bdnzflr                                                                                                                                                                                                                                              
                op str :  4*cr4+lt                                                                                                                                                                                                                                             
        Detail info:                                                                                                                                                                                                                                                           
                R regs :  3
                        10 (ctr)
                        43 (lr)
                        205 (rm)
                W regs :  1
                        10 (ctr)
                groups :  0
        Architecture-dependent info:
                branch code :  PPC_BC_LT
                branch hint :  PPC_BH_PLUS
                update cr0  :  false
                op count    :  1

                        type   :  PPC_OP_CRX
                        scale  :  4
                        reg    :  6 (cr4)
                        br cnd :  PPC_BC_LT

[aquynh]

in the "next" branch, we now have PPC_INS_BDNZFLR, but the output does not use it. should we make some alias here?

$ cstool -d ppc64be 4c100020    
 0  4c 10 00 20  bclr	0, cr4lt
	ID: 36 (bclr)
	op_count: 2
		operands[0].type: IMM = 0x0
		operands[1].type: REG = cr4lt

[PeterMatula]

Yes, there should be an alias. But I see you already made it in 30627ba since your last comment.

However, I'm not sure if everything gets set as it should. For example, before the 30627ba, e.g. at 8623090:

$ capstone-dumper -a ppc -m 32 -e big -b 0x1000 -c "4c100020"

Capstone version: 1280 (major: 5, minor: 0)

#0
        General info:
                id     :  36 (bclr)
                addr   :  1000
                size   :  4
                bytes  :  4c 10 00 20 
                mnem   :  bclr
                op str :  0, cr4lt
        Detail info:
                R regs :  3
                        3 (ctr)
                        5 (lr)
                        6 (rm)
                W regs :  1
                        3 (ctr)
                groups :  0
        Architecture-dependent info:
                branch code :  PPC_BC_INVALID
                branch hint :  PPC_BH_INVALID
                update cr0  :  false
                op count    :  2

                        type   :  PPC_OP_IMM
                        imm    :  0

                        type   :  PPC_OP_REG
                        reg    :  332 (cr4lt)

After 30627ba:

$ capstone-dumper -a ppc -m 32 -e big -b 0x1000 -c "4c100020"

Capstone version: 1280 (major: 5, minor: 0)

#0
        General info:
                id     :  46 (bdnzflr)
                addr   :  1000
                size   :  4
                bytes  :  4c 10 00 20 
                mnem   :  bdnzflr
                op str :  4*cr4+lt
        Detail info:
                R regs :  3
                        3 (ctr)
                        5 (lr)
                        6 (rm)
                W regs :  1
                        3 (ctr)
                groups :  0
        Architecture-dependent info:
                branch code :  PPC_BC_LT
                branch hint :  PPC_BH_PLUS
                update cr0  :  false
                op count    :  0

Operand information get lost. Is this intended? The bdnzflr has branch code set to PPC_BC_LT, but I don't see a way to get that it is cr4 (cr4lt in the bclr version) from the alias instruction. Am I missing something?

[aquynh]

I am confused with your output. Can you again explain the issue with cstool -d?

[PeterMatula]

8623090:

$ cstool -d ppc64be 4c100020
 0  4c 10 00 20  bclr   0, cr4lt
        ID: 36 (bclr)
        op_count: 2
                operands[0].type: IMM = 0x0
                operands[1].type: REG = cr4lt
        Branch code: 32754

30627ba:

$ cstool -d ppc64be 4c100020
 0  4c 10 00 20  bdnzflr        4*cr4+lt
        ID: 46 (bdnzflr)
        Branch code: 12
        Branch hint: 1

Basically, there were 2 operands: immediate 0 and register cr4lt. Now there are no operands, but branch code (12 = PPC_BC_LT) and branch hint (1 = PPC_BH_PLUS) are set. So, now it is possible to determine that the instruction is bdnzflr, and that PPC_BC_LT is happening, but I think information which cr[0-7]lt got lost.

[aquynh]

fixed now.

$ cstool -d ppc64be 4c100020
 0  4c 10 00 20  bdnzflr	4*cr4+lt
	ID: 46 (bdnzflr)
	op_count: 1
		operands[0].type: REG = cr4lt
	Branch code: 12
	Branch hint: 1
	Groups: jump 

[Rot127]

Closed with #2013