http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53590
Bug #: 53590
Summary: new compiler generates both SISD and SIMD instructions
for parallel operations of a "pure" function
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: ada
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: bauh...@futureapps.de
The Ada compiler of 4.8.0 on Intel will either
- produce 2 DIVSD and also 1 DIVPD (redundance), or
- produce 2 DIVSD (no measurable parallelism)
for the following function f that divides two pairs
of 64 bit FPT types and returns one pair.
The expectation is just 1 DIVPD.
Other versions of the Ada compiler, and the C compiler,
and the C++ compiler of the same version produce
1 DIVPD instruction, as expected.
The line marked --! below switches between the two cases
listed above.
package Autovect is
pragma Pure (Autovect);
type Fpt is new Long_Float;
type Vec is array (0 .. 1) of Fpt;
--pragma Machine_Attribute (Vec, "vector_type"); --!
--pragma Machine_Attribute (Vec, "may_alias");
function F (X0, X1, Y0, Y1 : Fpt) return Vec;
private
pragma Assert (Fpt'Size = 64);
pragma Assert (Vec'Alignment = 16);
end Autovect;
package body Autovect is
function F (X0, X1, Y0, Y1 : Fpt) return Vec is
begin
return (X0 / Y0, X1 / Y1);
end F;
end Autovect;
Result:
0000000000000010 <autovect__f>:
10: 66 0f 28 e8 movapd %xmm0,%xmm5
14: 66 0f 28 f2 movapd %xmm2,%xmm6
18: 66 0f 14 e9 unpcklpd %xmm1,%xmm5
1c: 66 0f 14 f3 unpcklpd %xmm3,%xmm6
20: f2 0f 5e c2 divsd %xmm2,%xmm0
24: 66 0f 28 e5 movapd %xmm5,%xmm4
28: 66 0f 5e e6 divpd %xmm6,%xmm4
2c: f2 0f 5e cb divsd %xmm3,%xmm1
30: 66 0f 29 64 24 e8 movapd %xmm4,-0x18(%rsp)
36: f2 0f 10 44 24 e8 movsd -0x18(%rsp),%xmm0
3c: f2 0f 10 4c 24 f0 movsd -0x10(%rsp),%xmm1
42: c3 retq
43: 66 66 66 66 2e 0f 1f data32 data32 data32 nopw
%cs:0x0(%rax,%rax,1)
4a: 84 00 00 00 00 00
With "vector_type" machine attribute:
0000000000000010 <autovect__f>:
10: f2 0f 5e c2 divsd %xmm2,%xmm0
14: f2 0f 5e cb divsd %xmm3,%xmm1
18: 66 0f 14 c1 unpcklpd %xmm1,%xmm0
1c: c3 retq
1d: 0f 1f 00 nopl (%rax)
gnatchop -r -w autovect.ada && gnatmake -gnatwa -W -O3 -fno-inline
-fomit-frame-pointer -msse3 -march=core2 -gnatp -gnata -v autovect.adb
splitting autovect.ada into:
autovect.ads
autovect.adb
GNATMAKE 4.8.0 20120525 (experimental)
Copyright (C) 1995-2012, Free Software Foundation, Inc.
"autovect.ali" being checked ...
-> "autovect.ads" time stamp mismatch
gcc -c -gnatwa -W -O3 -fno-inline -fomit-frame-pointer -msse3 -march=core2
-gnatp -gnata autovect.adb
End of compilation
Linux newnews 3.2.0-24-generic #39-Ubuntu SMP Mon May 21 16:52:17 UTC 2012
x86_64 x86_64 x86_64 GNU/Linux
Intel E6750
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/home/bauhaus/mine/libexec/gcc/x86_64-unknown-linux-gnu/4.8.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /home/bauhaus/src/gcc/trunk/configure
--prefix=/home/bauhaus/mine --disable-nls --disable-libstdcxx-pch
--enable-languages=c,ada,c++
Thread model: posix
gcc version 4.8.0 20120525 (experimental) (GCC)
