> -----Original Message-----
> From: MayShao-oc <mayshao...@zhaoxin.com>
> Sent: Tuesday, November 5, 2024 11:20 AM
> To: gcc-patches@gcc.gnu.org; hubi...@ucw.cz; Liu, Hongtao
> <hongtao....@intel.com>; ubiz...@gmail.com
> Cc: ti...@zhaoxin.com; silviaz...@zhaoxin.com; loui...@zhaoxin.com;
> cobec...@zhaoxin.com
> Subject: [PATCH] [x86_64] Add flag to control tight loops alignment opt
>
> Hi all:
> This patch add -malign-tight-loops flag to control pass_align_tight_loops.
> The motivation is that pass_align_tight_loops may cause performance
> regression in nested loops.
>
> The example code as follows:
>
> #define ITER 20000
> #define ITER_O 10
>
> int i, j,k;
> int array[ITER];
>
> void loop()
> {
> int i;
> for(k = 0; k < ITER_O; k++)
> for(j = 0; j < ITER; j++)
> for(i = 0; i < ITER; i++)
> {
> array[i] += j;
> array[i] += i;
> array[i] += 2*j;
> array[i] += 2*i;
> }
> }
>
> When I compile it with gcc -O1 loop.c, the output assembly as follows.
> It is not optimal, because of too many nops insert in the outer loop.
>
> 0000000000400540 <loop>:
> 400540: 48 83 ec 08 sub $0x8,%rsp
> 400544: bf 0a 00 00 00 mov $0xa,%edi
> 400549: b9 00 00 00 00 mov $0x0,%ecx
> 40054e: 8d 34 09 lea (%rcx,%rcx,1),%esi
> 400551: b8 00 00 00 00 mov $0x0,%eax
> 400556: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
> 40055d: 00 00 00 00
> 400561: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
> 400568: 00 00 00 00
> 40056c: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
> 400573: 00 00 00 00
> 400577: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
> 40057e: 00 00
> 400580: 89 ca mov %ecx,%edx
> 400582: 03 14 85 60 10 60 00 add 0x601060(,%rax,4),%edx
> 400589: 01 c2 add %eax,%edx
> 40058b: 01 f2 add %esi,%edx
> 40058d: 8d 14 42 lea (%rdx,%rax,2),%edx
> 400590: 89 14 85 60 10 60 00 mov %edx,0x601060(,%rax,4)
> 400597: 48 83 c0 01 add $0x1,%rax
> 40059b: 48 3d 20 4e 00 00 cmp $0x4e20,%rax
> 4005a1: 75 dd jne 400580 <loop+0x40>
>
> I benchmark this program in the intel Xeon, and find the optimization may
> cause a 40% performance regression (6.6B cycles VS 9.3B cycles).
> So I propose to add -malign-tight-loops flag to control tight loop
> optimization to avoid this, we could disalbe this optimization by default.
> Bootstrapped X86_64.
> Ok for trunk?
>
> BR
> Mayshao
>
> gcc/ChangeLog:
>
> * config/i386/i386-features.cc (ix86_align_tight_loops): New flag.
> * config/i386/i386.opt (malign-tight-loops): New option.
> * doc/invoke.texi (-malign-tight-loops): Document.
> ---
> gcc/config/i386/i386-features.cc | 4 +++-
> gcc/config/i386/i386.opt | 4 ++++
> gcc/doc/invoke.texi | 7 ++++++-
> 3 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/gcc/config/i386/i386-features.cc b/gcc/config/i386/i386-
> features.cc
> index e2e85212a4f..f9546e00b07 100644
> --- a/gcc/config/i386/i386-features.cc
> +++ b/gcc/config/i386/i386-features.cc
> @@ -3620,7 +3620,9 @@ public:
> /* opt_pass methods: */
> bool gate (function *) final override
> {
> - return optimize && optimize_function_for_speed_p (cfun);
> + return ix86_align_tight_loops
> + && optimize
> + && optimize_function_for_speed_p (cfun);
> }
>
> unsigned int execute (function *) final override diff --git
> a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt index
> 64c295d344c..ec41de192bc 100644
> --- a/gcc/config/i386/i386.opt
> +++ b/gcc/config/i386/i386.opt
> @@ -1266,6 +1266,10 @@ mlam=
> Target RejectNegative Joined Enum(lam_type) Var(ix86_lam_type)
> Init(lam_none) -mlam=[none|u48|u57] Instrument meta data position in
> user data pointers.
>
> +malign-tight-loops
> +Target Var(ix86_align_tight_loops) Init(0) Optimization Enable align
> +tight loops.
I'd like it to be on by default, so Init (1)?
> +
> Enum
> Name(lam_type) Type(enum lam_type) UnknownError(unknown lam
> type %qs)
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index
> 07920e07b4d..9ec1e1f0095 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -1510,7 +1510,7 @@ See RS/6000 and PowerPC Options.
> -mindirect-branch=@var{choice} -mfunction-return=@var{choice} -
> mindirect-branch-register -mharden-sls=@var{choice} -mindirect-branch-cs-
> prefix -mneeded -mno-direct-extern-access --munroll-only-small-loops -
> mlam=@var{choice}}
> +-munroll-only-small-loops -mlam=@var{choice} -malign-tight-loops}
>
> @emph{x86 Windows Options}
>
> @@ -36530,6 +36530,11 @@ LAM(linear-address masking) allows special
> bits in the pointer to be used for metadata. The default is @samp{none}. With
> @samp{u48}, pointer bits in positions 62:48 can be used for metadata; With
> @samp{u57}, pointer bits in positions 62:57 can be used for metadata.
> +
> +@opindex malign-tight-loops
> +@opindex mno-align-tight-loops
> +@item -malign-tight-loops
> +Controls tight loop alignment optimization.
> @end table
>
> @node x86 Windows Options
> --
> 2.27.0
