In this directory, I'm developing some test cases to study
some different options for Bochs performance improvements.
ideas:
write a loop or nested loop that takes some serious computing, that takes
maybe 15 seconds to run
benchmark on several machines
compile with optimization and understand the assembly
reimplement with switch stmt,function pointers, etc
benchmark again to measure the overhead of switch & function calls
test on several machines
reimplement with goto *label (gcc extension) style
benchmark again to measure the overhead of switch & function calls
test on several machines
or, by working more directly with the bochs source, we can test the
performance benefit of the "cut-and-paste native code" method on
a small scale.
---------------------------------------------------
Performance measurements
blur.c 1.2
-----------
estimated 1.43 million instructions per iteration (-O2)
(see blur-1.2-performance.gnumeric spreadsheet for details)
on Athlon 750 with no optimization, 4.0622 ms per iteration
on Athlon 750 with -O2, 1.795138 ms per iteration
on P2 350 with no optimization, 6.162224 ms per iteration
on P2 350 with -O2, 2.5258 ms per iteration
on Bochs with -O2 on Athlon 750, 478 ms per iteration
Conclusion
Bochs is about 270x slower than native code.
Try replacing the innermost loop of blur() with a function call
that does the same thing. Continue to compile with -O2.
if innermost loop is "sum += array[x2][y2]", 1.796 ms per iter.
if innermost loop is "blur_func (&sum, &array[x2][y2])", 3.526978 ms per iter.
function with three arguments: 3.784
function with four arguments: 4.390
function with five arguments: 4.879
what is the overhead in terms of instructions?
func_overhead(N, ARGS) for N instructions in the function, ARGS arguments
F(N, ARGS) = ?
1.5*ARGS to push them onto the stack
1 for the call
1 to set %esp back to normal
1.5*ARGS to load it into a register and maybe save the old register value
2 for leave & ret
= 4 + 3*ARGS instructions
This is ignoring the fact that some CISC instructions are much more expensive
than others.
However when you measure the time cost of the function call, it is
much larger.
Compare blur with no function call to blur with a call to a function with
2 arguments. 1.796 ms per iter versus 3.526978 ms per iter. The function
was called 142884 times, so the overhead of each function call is
12.1 ns. (In 12.1 ns this machine can execute between 9-10 instructions.)
I briefly tested the overhead of function calls of 2,3,4,5 arguments.
This should measure the difference between NO function call (inline function)
and a function call with N arguments.
2 args: 12.1 ns more than no function call
3 args: 13.91 ns more than no function call
4 args: 18.15 ns more than no function call
5 args: 21.58 ns more than no function call
Based on measurements, maybe the func_overhead should be
func_overhead(ARGS) = 4 + 2.5*ARGS instructions
Measure the cost of a switch statement.
With no switch statement, around 1.84 ms per iteration.
With 2 case switch+default, 2.3867 ms.
With 3 case switch+default, 3.739635 ms.
With 4 case switch, 3.341807 ms. (successive compares)
With 8 case switch, 2.241196 ms. (jump table)
With 16 case switch, 3.122807 ms. (jump table)
With 32 case switch, 3.080882 ms. (jump table)
Once you have enough cases that gcc creates a jump table, the
cost of a switch statement is about 7 instructions.
Try with a bunch of function calls in a switch stmt.
blur-O2 1.798693 ms
blur-O2-func 3.309882 ms
blur-O2-switch 3.101132 ms
blur-O2-switch-call 2.667050 ms
blur-O2-fnptr-switch 8.669802 ms
blur-O2-fnptr-table 6.010503 ms
;;;;;;;;;;;blur.c revision 1.2, compiled with -O2 -S;;;;;;;;;;;;
;; with some annotations by Bryce to figure out what is what.
.file "blur.c"
.version "01.01"
gcc2_compiled.:
.text
.align 4
.globl blur
.type blur,@function
blur:
pushl %ebp
movl %esp,%ebp
subl $36,%esp
pushl %edi
pushl %esi
pushl %ebx
movl $1,%eax
.p2align 4,,7
.L20:
movl $1,%edi
leal -1(%eax),%ebx
movl %ebx,-28(%ebp)
leal 1(%eax),%ebx
movl %ebx,-16(%ebp)
sall $9,%eax
movl %eax,-24(%ebp)
movl %ebx,-8(%ebp)
movl -28(%ebp),%ebx
movl %ebx,-32(%ebp)
sall $9,-32(%ebp)
.p2align 4,,7
.L24:
xorl %esi,%esi ; let %esi = sum
movl -28(%ebp),%ecx
leal 1(%edi),%ebx
movl %ebx,-20(%ebp)
cmpl -8(%ebp),%ecx
jg .L26
movl %ebx,-12(%ebp)
movl -16(%ebp),%ebx
movl %ebx,-4(%ebp)
movl -32(%ebp),%ebx
movl %ebx,-36(%ebp)
.p2align 4,,7
.L28:
leal -1(%edi),%edx ; let %edx = y2
cmpl -12(%ebp),%edx ; test if y2
jg .L27
;; build pointer in %eax to array[x2][y2]
movl -36(%ebp),%eax
addl $array,%eax
leal (%eax,%edx,4),%eax
.p2align 4,,7
.L32:
;; innermost loop. it has precomputed the endpoint in -20(%ebp)
addl (%eax),%esi ;; sum += (%eax)
addl $4,%eax ;; point %eax to next value
incl %edx ;; y2++
cmpl -20(%ebp),%edx ;; if y2<=max
jle .L32
.L27:
addl $512,-36(%ebp)
incl %ecx
cmpl -4(%ebp),%ecx
jle .L28
.L26:
movl -24(%ebp),%ebx
leal (%ebx,%edi,4),%eax
movl %esi,array2(%eax)
movl -20(%ebp),%edi
cmpl $126,%edi
jle .L24
movl -16(%ebp),%eax
cmpl $126,%eax
jle .L20
leal -48(%ebp),%esp
popl %ebx
popl %esi
popl %edi
leave
ret
.Lfe1:
.size blur,.Lfe1-blur
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
