[LLVMdev] LLVM x86 backend for Intel MIC : trying it out and questions

[Dmitry Mikushin]

Hello Elena,

Thanks for info! Since Knights Landing (KNL) is going to be shipped
also in form of host CPU, it will have to have open-source support :)
But given that KNL is only announced 1 month ago, we should expect up
to 1.5 years for it to become somewhat wide-spread, i.e. 2014-2015.
Meanwhile, I still hope to perform some KNC evaluation, so answers to
above questions are much appreciated!

Best,
- D.

2013/7/12 Demikhovsky, Elena <elena.demikhovsky at intel.com>:
> Hello Dmitry,
>
> I'm working on KNL backend and plan to push it to the open source once the ISA becomes public. We do not plan to support KNC architecture in open source.
>
> -  Elena
>
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Dmitry Mikushin
> Sent: Friday, July 12, 2013 01:51
> To: LLVM Developers Mailing List
> Subject: [LLVMdev] LLVM x86 backend for Intel MIC : trying it out and questions
>
> Dear all,
>
> I'm interested to analyse what could be done with current LLVM trunk to deliver basic Intel MIC support. Let's say, for basic level we'd want just scalar code execution, no threading, no zmm vectors.
> Attached verbose in text, but functionally very simple patch copy-pastes x86 and x86_64 backends into 32-bit and 64-bit K1OM. In the end of the message you can find how simple LLVM-generated programs could be compiled & executed on MIC device, using this patch.
>
> Could you please help finding answers to the following questions:
>
> 1) Is there actually a 32-bit mode for MIC? 32-bit ELFs are not recognized, so...
> 2) MIC ISA is 32-bit ISA (no SSE/MMX) plus 256-bit AVX-like vectors?
> 3) If 1 is "no" and 2 is "yes", then does MIC calling convention permit generation of programs that use only 32-bit x86 ISA? In other words, in common case, does calling convention require use of zmm registers (e.g. return double value) even in scalar programs?
>
> Thanks,
> - D.
>
> ===
>
> $ cat hello.c
> #include <stdio.h>
>
> int main()
> {
>         printf("Hello, Intel MIC!\n");
>         return 0;
> }
>
> $ PATH=$PATH:~/rpmbuild/CHROOT/opt/kernelgen/usr/bin clang -emit-llvm -c hello.c -o - | PATH=$PATH:~/forge/llvm/install/bin/ opt -O3 -S -o hello.ll $ cat hello.ll ; ModuleID = '<stdin>'
> target datalayout =
> "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
>
> @str = private unnamed_addr constant [18 x i8] c"Hello, Intel MIC!\00"
>
> ; Function Attrs: nounwind uwtable
> define i32 @main() #0 {
> entry:
>   %puts = tail call i32 @puts(i8* getelementptr inbounds ([18 x i8]* @str, i64 0, i64 0))
>   ret i32 0
> }
>
> ; Function Attrs: nounwind
> declare i32 @puts(i8* nocapture) #1
>
> attributes #0 = { nounwind uwtable "less-precise-fpmad"="false"
> "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"="true"
> "no-infs-fp-math"="false" "no-nans-fp-math"="false"
> "unsafe-fp-math"="false" "use-soft-float"="false" } attributes #1 = { nounwind }
>
> $ PATH=$PATH:~/forge/llvm/install/bin/ llc hello.ll -march=k1om64 -filetype=obj -o hello.mic.o $ objdump -d hello.mic.o
>
> hello.mic.o:     file format elf64-k1om
>
>
> Disassembly of section .text:
>
> 0000000000000000 <main>:
>    0:   55                      push   %rbp
>    1:   48 89 e5                mov    %rsp,%rbp
>    4:   bf 00 00 00 00          mov    $0x0,%edi
>    9:   e8 00 00 00 00          callq  e <main+0xe>
>    e:   31 c0                   xor    %eax,%eax
>   10:   5d                      pop    %rbp
>   11:   c3                      retq
>
> $ icc -mmic hello.mic.o -o hello
> x86_64-k1om-linux-ld: error in hello.mic.o(.eh_frame); no .eh_frame_hdr table will be created.
>
> $ /opt/intel/mic/bin/micnativeloadex ./hello Hello, Intel MIC!
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