Richard,
Here is the test code.
typedef short int16_t;
typedef unsigned short uint16_t;
void MemSet16(
int16_t *pBuf, /* Buffer address */
int16_t Val, /* Value to be set */
uint16_t Bytes /* Total size in bytes */
)
{
uint16_t Idx;
for(Idx=0; Idx<(Bytes>>1); Idx++)
*pBuf++ = Val;
}
I grepped insert_into_preds_of_block and found it is called only by
tree-ssa-pre.c. Actually, I am referring to RTL PRE pass in gcse.c
and lcm.c.
Before PRE:
;; Start of basic block ( 2) -> 3
;; bb 3 artificial_defs: { }
;; bb 3 artificial_uses: { u9(55){ }u10(57){ }u11(62){ }}
;; lr in 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 113 114 115
;; lr use 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 113 114
;; lr def 110 118 119 120 121
;; live in 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 113 114 115
;; live gen 110 118 119 120 121
;; live kill
;; Pred edge 2 [91.0%] (fallthru)
(note 34 33 35 3 [bb 3] NOTE_INSN_BASIC_BLOCK)
(insn 35 34 36 3 tst.c:4 (set (reg/f:SI 118)
(plus:SI (reg/v/f:SI 114 [ pBuf ])
(const_int 2 [0x2]))) 273 {addsi3} (nil))
(insn 36 35 37 3 tst.c:4 (set (reg:HI 119)
(plus:HI (reg:HI 113 [ D.3441 ])
(const_int -1 [0xffffffffffffffff]))) 276 {addhi3}
(expr_list:REG_DEAD (reg:HI 113 [ D.3441 ])
(nil)))
(insn 37 36 38 3 tst.c:4 (set (reg:SI 120)
(zero_extend:SI (reg:HI 119))) 1056 {zero_extendhisi2}
(expr_list:REG_DEAD (reg:HI 119)
(nil)))
(insn 38 37 39 3 tst.c:4 (set (reg:SI 121)
(ashift:SI (reg:SI 120)
(const_int 1 [0x1]))) 389 {ashlsi3} (expr_list:REG_DEAD (reg:SI 120)
(nil)))
(insn 39 38 43 3 tst.c:4 (set (reg/f:SI 110 [ D.3464 ])
(plus:SI (reg/f:SI 118)
(reg:SI 121))) 273 {addsi3} (expr_list:REG_DEAD (reg:SI 121)
(expr_list:REG_DEAD (reg/f:SI 118)
(nil))))
;; End of basic block 3 -> ( 4)
;; lr out 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; live out 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; Succ edge 4 [100.0%] (fallthru)
;; Start of basic block ( 4 3) -> 4
;; bb 4 artificial_defs: { }
;; bb 4 artificial_uses: { u18(55){ }u19(57){ }u20(62){ }}
;; lr in 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; lr use 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; lr def 114 122
;; live in 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; live gen 114 122
;; live kill
;; Pred edge 4 [91.0%]
;; Pred edge 3 [100.0%] (fallthru)
(code_label 43 39 40 4 3 "" [1 uses])
(note 40 43 41 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
(insn 41 40 42 4 tst.c:14 (set (mem:HI (reg/v/f:SI 114 [ pBuf ]) [2 *pBuf+0 S2
A16])
(reg/v:HI 115 [ Val ])) 236 {*movhhi} (nil))
(insn 42 41 44 4 tst.c:14 (set (reg/v/f:SI 114 [ pBuf ])
(plus:SI (reg/v/f:SI 114 [ pBuf ])
(const_int 2 [0x2]))) 273 {addsi3} (nil))
(insn 44 42 45 4 tst.c:13 (set (reg:BI 122)
(ne:BI (reg/v/f:SI 114 [ pBuf ])
(reg/f:SI 110 [ D.3464 ]))) 1006 {cmp_simode} (nil))
(jump_insn 45 44 48 4 tst.c:13 (set (pc)
(if_then_else (ne (reg:BI 122)
(const_int 0 [0x0]))
(label_ref 43)
(pc))) 1085 {cbranchbi4} (expr_list:REG_DEAD (reg:BI 122)
(expr_list:REG_BR_PROB (const_int 9100 [0x238c])
(expr_list:REG_PRED_WIDTH (const_int 4 [0x4])
(nil))))
-> 43)
;; End of basic block 4 -> ( 4 5)
;; lr out 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; live out 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
After PRE:
;; Start of basic block ( 2) -> 3
;; bb 3 artificial_defs: { }
;; bb 3 artificial_uses: { u9(55){ }u10(57){ }u11(62){ }}
;; lr in 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 113 114 115
;; lr use 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 113 114
;; lr def 110 118 119 120 121
;; live in 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 113 114 115
;; live gen 110 118 119 120 121
;; live kill
;; Pred edge 2 [91.0%] (fallthru)
(note 34 33 35 3 [bb 3] NOTE_INSN_BASIC_BLOCK)
(insn 35 34 53 3 tst.c:4 (set (reg/f:SI 123 [ pBuf ])
(plus:SI (reg/v/f:SI 114 [ pBuf ])
(const_int 2 [0x2]))) 273 {addsi3} (nil))
(insn 53 35 36 3 tst.c:4 (set (reg/f:SI 118)
(reg/f:SI 123 [ pBuf ])) -1 (nil))
(insn 36 53 37 3 tst.c:4 (set (reg:HI 119)
(plus:HI (reg:HI 113 [ D.3441 ])
(const_int -1 [0xffffffffffffffff]))) 276 {addhi3}
(expr_list:REG_DEAD (reg:HI 113 [ D.3441 ])
(nil)))
(insn 37 36 38 3 tst.c:4 (set (reg:SI 120)
(zero_extend:SI (reg:HI 119))) 1056 {zero_extendhisi2}
(expr_list:REG_DEAD (reg:HI 119)
(nil)))
(insn 38 37 39 3 tst.c:4 (set (reg:SI 121)
(ashift:SI (reg:SI 120)
(const_int 1 [0x1]))) 389 {ashlsi3} (expr_list:REG_DEAD (reg:SI 120)
(nil)))
(insn 39 38 43 3 tst.c:4 (set (reg/f:SI 110 [ D.3464 ])
(plus:SI (reg/f:SI 118)
(reg:SI 121))) 273 {addsi3} (expr_list:REG_DEAD (reg:SI 121)
(expr_list:REG_DEAD (reg/f:SI 118)
(nil))))
;; End of basic block 3 -> ( 4)
;; lr out 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; live out 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; Succ edge 4 [100.0%] (fallthru)
;; Start of basic block ( 6 3) -> 4
;; bb 4 artificial_defs: { }
;; bb 4 artificial_uses: { u18(55){ }u19(57){ }u20(62){ }}
;; lr in 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; lr use 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; lr def 114 122
;; live in 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; live gen 114 122
;; live kill
;; Pred edge 6 [100.0%] (fallthru)
;; Pred edge 3 [100.0%] (fallthru)
(code_label 43 39 40 4 3 "" [0 uses])
(note 40 43 41 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
(insn 41 40 51 4 tst.c:14 (set (mem:HI (reg/v/f:SI 114 [ pBuf ]) [2 *pBuf+0 S2
A16])
(reg/v:HI 115 [ Val ])) 236 {*movhhi} (nil))
(insn 51 41 44 4 tst.c:14 (set (reg/v/f:SI 114 [ pBuf ])
(reg/f:SI 123 [ pBuf ])) -1 (expr_list:REG_EQUAL (plus:SI (reg/v/f:SI
114 [ pBuf ])
(const_int 2 [0x2]))
(nil)))
(insn 44 51 45 4 tst.c:13 (set (reg:BI 122)
(ne:BI (reg/v/f:SI 114 [ pBuf ])
(reg/f:SI 110 [ D.3464 ]))) 1006 {cmp_simode} (nil))
(jump_insn 45 44 55 4 tst.c:13 (set (pc)
(if_then_else (ne (reg:BI 122)
(const_int 0 [0x0]))
(label_ref:SI 55)
(pc))) 1085 {cbranchbi4} (expr_list:REG_DEAD (reg:BI 122)
(expr_list:REG_BR_PROB (const_int 9100 [0x238c])
(expr_list:REG_PRED_WIDTH (const_int 4 [0x4])
(nil))))
-> 55)
;; End of basic block 4 -> ( 6 5)
;; lr out 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; live out 55 [r55] 57 [r57] 62 [__arg_pointer_register__] 110 114 115
;; Succ edge 6 [91.0%]
;; Succ edge 5 [9.0%] (fallthru)
;; Start of basic block ( 4) -> 6
;; bb 6 artificial_defs: { }
;; bb 6 artificial_uses: { u-1(55){ }u-1(57){ }u-1(62){ }}
;; Pred edge 4 [91.0%]
(code_label 55 45 54 6 4 "" [1 uses])
(note 54 55 52 6 [bb 6] NOTE_INSN_BASIC_BLOCK)
(insn 52 54 48 6 (set (reg/f:SI 123 [ pBuf ])
(plus:SI (reg/v/f:SI 114 [ pBuf ])
(const_int 2 [0x2]))) 273 {addsi3} (nil))
;; End of basic block 6 -> ( 4)
Thanks,
Bingfeng
> -----Original Message-----
> From: Richard Guenther [mailto:richard.guent...@gmail.com]
> Sent: 29 September 2010 13:33
> To: Bingfeng Mei
> Cc: gcc@gcc.gnu.org
> Subject: Re: Worse code generated by PRE
>
> On Wed, Sep 29, 2010 at 2:16 PM, Bingfeng Mei <b...@broadcom.com> wrote:
> > Hello,
> > I have been examining a significant performance regression
> > between 4.5 and 4.4 in our port. I found that Partial Redundancy
> > Elimination introduced in 4.5 causes the issue. The following
> > pseudo code explains the problem:
> >
> > BB 3:
> > r118 <- r114 + 2
> >
> > BB 4:
> > R114 <- r114 + 2
> > ...
> > Conditional jump to BB 4
> >
> > After PRE
> >
> > BB 3:
> > r123 <- r114 + 2
> > r118 <- r123
> >
> > BB 4:
> > r114 <- r123
> > conditional jump to BB 5
> >
> > BB5:
> > r123 <- r114 + 2
> > jump to BB 4
> >
> >
> > A simple loop (BB 4) is divided into two basic blocks (BB 4 & 5).
> > An extra jump instruction is introduced. On some targets, this
> > jump can be removed by bb-reorder pass. On our target, it cannot
> > be reordered due to complex doloop_end pattern we generate later.
> > Additionally, since bb-reorder is done in very late phase, the code
> > miss some optimization opportunity such as auto_inc_dec. I don't
> > see any benefit here to do PRE like this. Maybe we should exclude
> > such case in the first place? I read the relevant text in
> > "Advanced Compiler Design Implementation", the example used is linear
> > CFG and it doesn't mention how to handle loop case.
>
> PRE basically sinks the computation into the latch block (possibly
> creating that). Note that without a testcase it's hard to tell whether
> this is ok in general. PRE tries to avoid generation of new induction
> variables and cross-iteration data-dependences, see
> insert_into_preds_of_block.
> Note that 4.4 in principle performs the same optimization (you might
> figure that PRE in 4.4 is generally disabled for -Os but enabled in 4.5,
> but only for hot execution traces following existing practice to tune
> code-size/performance on a fine-grained basis).
>
> Richard.
>
> > Any suggestion is greatly appreciated.
> >
> > Thanks,
> > Bingfeng Mei
> >
> >
> >
> >
> >
> >
> >
> >
