Inefficient use of 64-bit addresses in Clang

Mark Geisert mark@maxrnd.com
Mon Aug 12 09:20:00 GMT 2019


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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