It is expression canonicalization issue.
the source is:
c = b*60;
d = c+44;
for (i = 0; i< b; i++)
{
x = x+d;
*a++=x;
}
Before PRE, due to the copy propagation and expression canonicalization, it
has become:
for (i = 0; i < b; i++) {
x = (x + (b*60)) + 44;
}
The PRE, only (b*60) got move out of the loop.
To solve the problem, someone needs to understand what the expression
canonicalization could be done to avoid this suboptimal form.
if the loop has the following form, PRE would be able to undo the copy
propagation.
for (i = 0; i < b; i++) {
x = x + ((b*60)) + 44);
}
- Shin
On Thu, Jul 12, 2012 at 3:28 PM, Yiran Wang <yiran.w...@gmail.com> wrote:
> Hi All,
>
> It looks like strength reduction is not optimal for the following example?
>
> 7 instructions per iteration is used, but 4 (or 5 without LFTR) are
> necessary.
>
> Best Regards,
> Yiran Wang
>
> bash-4.0$ cat x.c
> int foo(int x, int b, int *__restrict a)
> {
> int i;
> int c,d ;
> c = b*60;
> d = c+44;
>
> for (i = 0; i< b; i++)
> {
> x = x+d;
> *a++=x;
> }
> return x;
> }
> bash-4.0$ /opt/open64tr/bin/opencc -c -O3 -keep x.c
> -Wb,-trlow,-tt25:0xffffffff -OPT:unroll_times_max=1 -march=barcelona
> bash-4.0$ cat x.s
> # /opt/open64tr/lib/gcc-lib/x86_64-open64-linux/5.0/be::5.0
>
> #-----------------------------------------------------------
> # Compiling x.c (x.I)
> #-----------------------------------------------------------
>
> #-----------------------------------------------------------
> # Options:
> #-----------------------------------------------------------
> # Target:Barcelona, ISA:ISA_1, Endian:little, Pointer Size:32
> # -O3 (Optimization level)
> # -g0 (Debug level)
> # -m2 (Report advisories)
> #-----------------------------------------------------------
>
> int foo(int x, int b, int *__restrict a)
> {
> int i;
> int c,d ;
> c = b*60;
> d = c+44;
>
> for (i = 0; i< b; i++)
> {
> x = x+d;
> *a++=x;
> }
> return x;
> }
> bash-4.0$ /opt/open64tr/bin/opencc -c -O3 -keep x.c
> -Wb,-trlow,-tt25:0xffffffff -OPT:unroll_times_max=1 -march=barcelona
> bash-4.0$ cat x.s
> # /opt/open64tr/lib/gcc-lib/x86_64-open64-linux/5.0/be::5.0
>
> #-----------------------------------------------------------
> # Compiling x.c (x.I)
> #-----------------------------------------------------------
>
> #-----------------------------------------------------------
> # Options:
> #-----------------------------------------------------------
> # Target:Barcelona, ISA:ISA_1, Endian:little, Pointer Size:32
> # -O3 (Optimization level)
> # -g0 (Debug level)
> # -m2 (Report advisories)
> #-----------------------------------------------------------
>
> .text
> .align 2
> .section .text
> .p2align 5,,
>
> # Program Unit: foo
> .globl foo
> .type foo, @function
> foo: # 0x0
> # .frame %esp, 16, %esp
> # _temp_gra_spill0 = 0
> .loc 1 2 0
> # 1 int foo(int x, int b, int *__restrict a)
> # 2 {
> .LBB1_foo:
> pushl %ebp # [0]
> pushl %ebx # [3]
> pushl %edi # [6]
> addl $-16,%esp # [9]
> movl 36(%esp),%edi # [10] b
> leal -1(%edi),%eax # [13]
> testl %eax,%eax # [14]
> jl .Lt_0_2818 # [15]
> .LBB2_foo:
> movl %edi,%ebp # [0]
> .loc 1 8 0
> # 4 int c,d ;
> # 5 c = b*60;
> # 6 d = c+44;
> # 7
> # 8 for (i = 0; i< b; i++)
> movl %edi,%ecx # [0]
> movl 32(%esp),%ebx # [0] x
> movl %ecx,0(%esp) # [1] _temp_gra_spill0
> imull $60,%ebp # [1]
> movl 40(%esp),%eax # [1] a
> xorl %edx,%edx # [2]
> .p2align 5,,31
> .Lt_0_3586:
> #<loop> Loop body line 8, nesting depth: 1, estimated iterations: 1000
> .loc 1 11 0
> # 9 {
> # 10 x = x+d;
> # 11 *a++=x;
> addl $1,%edx # [0]
> .loc 1 10 0
> addl %ebp,%ebx # [0]
> .loc 1 11 0
> addl $4,%eax # [0]
> .loc 1 10 0
> addl $44,%ebx # [1]
> .loc 1 11 0
> cmpl %edi,%edx # [1]
> movl %ebx,-4(%eax) # [2] id:17
> jl .Lt_0_3586 # [2]
> .Lt_0_4098:
> .loc 1 13 0
> # 12 }
> # 13 return x;
> movl %ebx,%eax # [0]
> addl $16,%esp # [0]
> popl %edi # [1]
> popl %ebx # [4]
> popl %ebp # [7]
> ret # [7]
> .p2align 5,,31
> .Lt_0_2818:
> .loc 1 11 0
> movl 32(%esp),%eax # [0] x
> .loc 1 13 0
> addl $16,%esp # [0]
> popl %edi # [1]
> popl %ebx # [4]
> popl %ebp # [7]
> ret # [7]
> .LDWend_foo:
> .size foo, .LDWend_foo-foo
> .section .text
> .align 4
>
> .section .eh_frame, "a",@progbits
> .LEHCIE:
> .4byte .LEHCIE_end - .LEHCIE_begin
> .LEHCIE_begin:
> .4byte 0x0
> .byte 0x01, 0x00, 0x01, 0x7c, 0x08, 0x0c, 0x04, 0x04
> .byte 0x88, 0x01
> .align 4
> .LEHCIE_end:
>
> .section .debug_line, ""
> .section .note.GNU-stack,"",@progbits
> .ident "#Open64 Compiler Version 5.0 : x.c compiled with : -O3
> -OPT:unroll_times_max=1 -march=barcelona -msse2 -msse3 -mno-3dnow
> -mno-sse4a -mno-ssse3 -mno-sse41 -mno-sse42 -mno-aes -mno-pclmul -mno-avx
> -mno-xop -mno-fma -mno-fma4 -m32"
>
>
>
> .text
> .align 2
> .section .text
> .p2align 5,,
>
> # Program Unit: foo
> .globl foo
> .type foo, @function
> foo: # 0x0
> # .frame %esp, 16, %esp
> # _temp_gra_spill0 = 0
> .loc 1 2 0
> # 1 int foo(int x, int b, int *__restrict a)
> # 2 {
> .LBB1_foo:
> pushl %ebp # [0]
> pushl %ebx # [3]
> pushl %edi # [6]
> addl $-16,%esp # [9]
> movl 36(%esp),%edi # [10] b
> leal -1(%edi),%eax # [13]
> testl %eax,%eax # [14]
> jl .Lt_0_2818 # [15]
> .LBB2_foo:
> movl %edi,%ebp # [0]
> .loc 1 8 0
> # 4 int c,d ;
> # 5 c = b*60;
> # 6 d = c+44;
> # 7
> # 8 for (i = 0; i< b; i++)
> movl %edi,%ecx # [0]
> movl 32(%esp),%ebx # [0] x
> movl %ecx,0(%esp) # [1] _temp_gra_spill0
> imull $60,%ebp # [1]
> movl 40(%esp),%eax # [1] a
> xorl %edx,%edx # [2]
> .p2align 5,,31
> .Lt_0_3586:
> #<loop> Loop body line 8, nesting depth: 1, estimated iterations: 1000
> .loc 1 11 0
> # 9 {
> # 10 x = x+d;
> # 11 *a++=x;
> addl $1,%edx # [0]
> .loc 1 10 0
> addl %ebp,%ebx # [0]
> .loc 1 11 0
> addl $4,%eax # [0]
> .loc 1 10 0
> addl $44,%ebx # [1]
> .loc 1 11 0
> cmpl %edi,%edx # [1]
> movl %ebx,-4(%eax) # [2] id:17
> jl .Lt_0_3586 # [2]
> .Lt_0_4098:
> .loc 1 13 0
> # 12 }
> # 13 return x;
> movl %ebx,%eax # [0]
> addl $16,%esp # [0]
> popl %edi # [1]
> popl %ebx # [4]
> popl %ebp # [7]
> ret # [7]
> .p2align 5,,31
> .Lt_0_2818:
> .loc 1 11 0
> movl 32(%esp),%eax # [0] x
> .loc 1 13 0
> addl $16,%esp # [0]
> popl %edi # [1]
> popl %ebx # [4]
> popl %ebp # [7]
> ret # [7]
> .LDWend_foo:
> .size foo, .LDWend_foo-foo
> .section .text
> .align 4
>
> .section .eh_frame, "a",@progbits
> .LEHCIE:
> .4byte .LEHCIE_end - .LEHCIE_begin
> .LEHCIE_begin:
> .4byte 0x0
> .byte 0x01, 0x00, 0x01, 0x7c, 0x08, 0x0c, 0x04, 0x04
> .byte 0x88, 0x01
> .align 4
> .LEHCIE_end:
>
> .section .debug_line, ""
> .section .note.GNU-stack,"",@progbits
> .ident "#Open64 Compiler Version 5.0 : x.c compiled with : -O3
> -OPT:unroll_times_max=1 -march=barcelona -msse2 -msse3 -mno-3dnow
> -mno-sse4a -mno-ssse3 -mno-sse41 -mno-sse42 -mno-aes -mno-pclmul -mno-avx
> -mno-xop -mno-fma -mno-fma4 -m32"
>
>
>
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>
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