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

Dmitry Mikushin dmitry at kernelgen.org
Thu Jul 11 15:51:01 PDT 2013

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
$ 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!
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