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.
+
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
