Re: Can register rename pass rename a callee-saved register?

[Mohamed Shafi]

2008/6/19 Ian Lance Taylor <[EMAIL PROTECTED]>:
> "Mohamed Shafi" <[EMAIL PROTECTED]> writes:
>
>> Before register renaming pass, callee registers was being used in the
>> body of the code. Hence function prologue saved the register and
>> epilogue restored the register. But register renaming pass removed
>> this particular callee saved register.The output and code generation
>> is proper, but there is an unnecessary save and restore of a callee
>> saved register in the prologue and epilogue even though the reference
>> of the callee saved register has been removed by the renaming pass.
>> I am using the prologue/epilogue patterns instead of the target macros.
>
> Which version of gcc?  I was under the impression that this
> longstanding buglet was cleaned up by the dataflow work.
>

   I am doing a port in gcc 4.1.2. The register is actually replaced
by register copy-propagation optimization pass.
   Here is the rtl dumps before .rnreg (the relevant portions)


(insn/f 42 41 43 0 (set (mem:HI (reg/f:HI 12 R12) [0 S2 A16])
        (reg:HI 4 R4)) -1 (nil)
    (expr_list:REG_DEAD (reg:HI 4 R4)
        (nil)))

(note 43 42 9 0 NOTE_INSN_PROLOGUE_END)

(note 9 43 14 0 NOTE_INSN_FUNCTION_BEG)

(insn 14 9 37 0 (set (reg:HI 4 R4 [orig:26+2 ] [26])
        (reg:HI 0 R0 [ pExtern ])) 1 {*movhi_internal}
(insn_list:REG_DEP_ANTI 16 (nil))
    (expr_list:REG_DEAD (reg:HI 0 R0 [ pExtern ])
        (expr_list:REG_NO_CONFLICT (reg/v:SI 0 R0 [orig:23 pExtern ] [23])
            (nil))))

(insn 37 14 18 0 (set (reg:HI 8 R8)
        (const_int 42 [0x2a])) 1 {*movhi_internal} (nil)
    (nil))

(insn 18 37 38 0 (set (unspec:HI [
                (reg:HI 8 R8)
            ] 2)
        (unspec_volatile:HI [
                (reg:HI 4 R4 [orig:26+2 ] [26])
            ] 6)) 6 {out} (insn_list:REG_DEP_TRUE 17
(insn_list:REG_DEP_ANTI 14 (insn_list:REG_DEP_TRUE 7
(insn_list:REG_DEP_TRUE 6 (nil)))))
    (expr_list:REG_DEAD (reg:HI 8 R8)
        (expr_list:REG_DEAD (reg:HI 4 R4 [orig:26+2 ] [26])
            (nil))))


And this is the after the optimization pass


insn 18: replaced reg 4 with 0

.....
(insn/f 42 41 43 0 (set (mem:HI (reg/f:HI 12 R12) [0 S2 A16])
        (reg:HI 4 R4)) 1 {*movhi_internal} (nil)
    (expr_list:REG_DEAD (reg:HI 4 R4)
        (nil)))

(note 43 42 9 0 NOTE_INSN_PROLOGUE_END)

(note 9 43 37 0 NOTE_INSN_FUNCTION_BEG)

(insn 37 9 18 0 (set (reg:HI 8 R8)
        (const_int 42 [0x2a])) 1 {*movhi_internal} (nil)
    (nil))

(insn 18 37 38 0 (set (unspec:HI [
                (reg:HI 8 R8)
            ] 2)
        (unspec_volatile:HI [
                (reg:HI 0 R0 [orig:26+2 ] [26])
            ] 6)) 6 {out} (insn_list:REG_DEP_TRUE 17
(insn_list:REG_DEP_ANTI 14 (insn_list:REG_DEP_TRUE 7
(insn_list:REG_DEP_TRUE 6 (nil)))))
    (expr_list:REG_DEAD (reg:HI 8 R8)
        (expr_list:REG_DEAD (reg:HI 0 R0 [orig:26+2 ] [26])
            (nil))))




>> So is the rename pass allowed to rename a callee saved register? Where
>> might this going wrong?
>
> If this is the buglet I'm thinking of, the resulting code does work,
> despite being suboptimal.  It just does an unnecessary save and
> restore.

  The resulting code is proper except for the unnecessary save and
> restore.
>
> Ian
>