[FFmpeg-devel] [PATCH 1/2] avfilter/vf_histogram: clean up code

[zhilizhao]

> On Dec 15, 2019, at 12:35 AM, Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:
> 
> Am Sa., 14. Dez. 2019 um 08:28 Uhr schrieb zhilizhao <quinkblack at foxmail.com>:
> 
>>> On Dec 13, 2019, at 10:00 PM, Paul B Mahol <onemda at gmail.com> wrote:
>>> 
>>> Please provide some explanation.
>> 
>> No functional changes, just use temporary variables to make code more readable,
> 
>> and may improve a little bit of performance.
> 
> Such a claim needs numbers and / or a testcase.

It may or may not improve performance depends on compiler optimization, and I don’t
think such small change can lead to measurable difference.

Assemble results on aarch64. Less deferences lead to smaller inner loop, but it doesn’t
mean much.


Before:
.LBB1_11:                               //   Parent Loop BB1_4 Depth=1
                                        // =>  This Inner Loop Header: Depth=2
	//DEBUG_VALUE: filter_frame:i <- 0
	//DEBUG_VALUE: dst_w <- $w11
	//DEBUG_VALUE: dst_h <- $w25
	//DEBUG_VALUE: filter_frame:k <- $x19
	//DEBUG_VALUE: filter_frame:out <- $x20
	//DEBUG_VALUE: filter_frame:outlink <- $x28
	//DEBUG_VALUE: filter_frame:h <- $x22
	//DEBUG_VALUE: filter_frame:i <- $w26
	.loc	15 272 37 is_stmt 1     // src/libavfilter/vf_histogram.c:272:37
	ldr	x8, [x23, #48]
	.loc	15 272 17 is_stmt 0     // src/libavfilter/vf_histogram.c:272:17
	ldrb	w1, [x27]
	.loc	15 272 59               // src/libavfilter/vf_histogram.c:272:59
	sxtw	x10, w26
	.loc	15 272 17               // src/libavfilter/vf_histogram.c:272:17
	mov	x2, x21
	.loc	15 272 52               // src/libavfilter/vf_histogram.c:272:52
	add	x8, x8, x19, lsl #5
	ldrsw	x8, [x8, #24]
	.loc	15 272 24               // src/libavfilter/vf_histogram.c:272:24
	ldr	x9, [x20, x8, lsl #3]
	.loc	15 273 28 is_stmt 1     // src/libavfilter/vf_histogram.c:273:28
	add	x8, x20, x8, lsl #2
	ldrsw	x8, [x8, #64]
	.loc	15 272 59               // src/libavfilter/vf_histogram.c:272:59
	nop
	madd	x0, x8, x10, x9
	.loc	15 272 17 is_stmt 0     // src/libavfilter/vf_histogram.c:272:17
	bl	memset
.Ltmp74:
	.loc	15 271 38 is_stmt 1     // src/libavfilter/vf_histogram.c:271:38
	add	w26, w26, #1            // =1
.Ltmp75:
	//DEBUG_VALUE: filter_frame:i <- $w26
	.loc	15 271 13 is_stmt 0     // src/libavfilter/vf_histogram.c:271:13
	cmp	w25, w26
	b.ne	.LBB1_11
-----------------------------------------------------------------------------------
After:
.LBB1_11:                               //   Parent Loop BB1_4 Depth=1
                                        // =>  This Inner Loop Header: Depth=2
	//DEBUG_VALUE: dst_h <- $w29
	//DEBUG_VALUE: filter_frame:outlink <- [DW_OP_plus_uconst 16] [$sp+0]
	//DEBUG_VALUE: filter_frame:i <- 0
	//DEBUG_VALUE: data <- $x21
	//DEBUG_VALUE: dst_w <- $w9
	//DEBUG_VALUE: filter_frame:k <- $x19
	//DEBUG_VALUE: filter_frame:out <- $x20
	//DEBUG_VALUE: filter_frame:h <- $x23
	//DEBUG_VALUE: filter_frame:i <- undef
	.loc	15 275 17 is_stmt 1     // src/libavfilter/vf_histogram.c:275:17
	ldrb	w1, [x28]
	mov	x0, x21
	mov	x2, x22
	bl	memset
.Ltmp79:
	//DEBUG_VALUE: filter_frame:i <- [DW_OP_plus_uconst 1, DW_OP_stack_value] undef
	.loc	15 274 27               // src/libavfilter/vf_histogram.c:274:27
	subs	x29, x29, #1            // =1
.Ltmp80:
	add	x21, x21, x27
.Ltmp81:
	.loc	15 274 13 is_stmt 0     // src/libavfilter/vf_histogram.c:274:13
	b.ne	.LBB1_11


> 
> Carl Eugen
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