Inefficient use of 64-bit addresses in Clang

Inefficient use of 64-bit addresses in Clang

[Agner Fog]

Clang is using 64-bit absolute addresses when accessing static data in 
64-bit mode. This is inefficient because it requires an extra 10-bytes 
long instruction for loading an address into a register every time it 
needs to access static data. All other compilers use relative addresses.

Example:

> #include <immintrin.h>
>
> __m128d test (__m128d a) {
>     __m128d b = _mm_add_pd(a, _mm_set1_pd(1.5));
>     __m128d c = _mm_mul_pd(b, _mm_set1_pd(2.5));
>     return c;
> }

Assembly output:

> .LCPI0_0:
>     .quad    4609434218613702656     # double 1.5
>     .quad    4609434218613702656     # double 1.5
> .LCPI0_1:
>     .quad    4612811918334230528     # double 2.5
>     .quad    4612811918334230528     # double 2.5
>     .text
>     .globl    _Z4testDv2_d
>     .p2align    4, 0x90
> _Z4testDv2_d:                           # @_Z4testDv2_d
> # BB#0:
>     vmovapd    (%rcx), %xmm0
>     movabsq    $.LCPI0_0, %rax
>     vaddpd    (%rax), %xmm0, %xmm0
>     movabsq    $.LCPI0_1, %rax
>     vmulpd    (%rax), %xmm0, %xmm0
>     retq

Linux Clang uses 32-bit relative addresses:

>     vaddpd    .LCPI0_0(%rip), %xmm0, %xmm0
>     vmulpd    .LCPI0_1(%rip), %xmm0, %xmm0
>     retq


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Re: Inefficient use of 64-bit addresses in Clang

[Mark Geisert]

Agner Fog wrote:
> Clang is using 64-bit absolute addresses when accessing static data in 64-bit 
> mode. This is inefficient because it requires an extra 10-bytes long instruction 
> for loading an address into a register every time it needs to access static 
> data. All other compilers use relative addresses.
> 
> Example:
> 
>> #include <immintrin.h>
>>
>> __m128d test (__m128d a) {
>>     __m128d b = _mm_add_pd(a, _mm_set1_pd(1.5));
>>     __m128d c = _mm_mul_pd(b, _mm_set1_pd(2.5));
>>     return c;
>> }
> 
> Assembly output:
> 
>> .LCPI0_0:
>>     .quad    4609434218613702656     # double 1.5
>>     .quad    4609434218613702656     # double 1.5
>> .LCPI0_1:
>>     .quad    4612811918334230528     # double 2.5
>>     .quad    4612811918334230528     # double 2.5
>>     .text
>>     .globl    _Z4testDv2_d
>>     .p2align    4, 0x90
>> _Z4testDv2_d:                           # @_Z4testDv2_d
>> # BB#0:
>>     vmovapd    (%rcx), %xmm0
>>     movabsq    $.LCPI0_0, %rax
>>     vaddpd    (%rax), %xmm0, %xmm0
>>     movabsq    $.LCPI0_1, %rax
>>     vmulpd    (%rax), %xmm0, %xmm0
>>     retq
> 
> Linux Clang uses 32-bit relative addresses:
> 
>>     vaddpd    .LCPI0_0(%rip), %xmm0, %xmm0
>>     vmulpd    .LCPI0_1(%rip), %xmm0, %xmm0
>>     retq

This kind of bug report should probably go to the Clang bugtracker at 
https://bugs.llvm.org .  On that page you could enter "clang cygwin" in the 
Quick Search field and see what known issues there are.

Clang is packaged for Cygwin by a very industrious and busy maintainer, but no 
development on Clang is being done here (to my knowledge).  So something like a 
Cygwin packaging error that breaks Clang on Cygwin would be on-topic for this 
list, but a problem at code generation level should go to the Clang developers.

..mark

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Re: Inefficient use of 64-bit addresses in Clang

[Agner Fog]

It's a difference in memory model.

clang 6.0.0 under ubuntu with --target=x86_64-pc-cygwin gives relative 
addresses, unless you specify -mcmodel=large.

Cygwin clang with -mcmodel=small does the right thing: use relative 
addresses.

The -mcmodel=small option appears to work differently for Linux and for 
Windows targets. I cannot find any documentation of this difference. See:

https://bugs.llvm.org/show_bug.cgi?id=42983


On 12/08/2019 11.45, falk.tannhauser@free.fr wrote:
> References: <578eb489-9391-9009-82ad-676eeb4c1c92@agner.org>
> In-Reply-To: <578eb489-9391-9009-82ad-676eeb4c1c92@agner.org>
>
> Could the different behaviour between Cygwin and Linux simply be due to different Clang versions?
> The current version under Cygwin is 5.0.1, while the latest version available under Linux
> appears to be 8.0.1 .
>
> Falk
>
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Re: Inefficient use of 64-bit addresses in Clang

[falk.tannhauser]

References: <578eb489-9391-9009-82ad-676eeb4c1c92@agner.org>
In-Reply-To: <578eb489-9391-9009-82ad-676eeb4c1c92@agner.org>

Could the different behaviour between Cygwin and Linux simply be due to different Clang versions?
The current version under Cygwin is 5.0.1, while the latest version available under Linux
appears to be 8.0.1 .

Falk

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