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

Demikhovsky, Elena elena.demikhovsky at intel.com
Fri Jul 12 13:12:35 PDT 2013

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?

- 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 =
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 {
  %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!
Intel Israel (74) Limited

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