介绍
目前clang/llvm编译器已经支持OpenMP，目前，OpenMP 3.1已经被clang/llvm 3.7完全支持。
方法
这是一个openmp程序
#include <omp.h>
#include <stdio.h>
int main() {
#pragma omp parallel
printf("Hello from thread %d, nthreads %d\n", omp_get_thread_num(), omp_get_num_threads());
}
将更改程序转化为相应的IR代码：
clang -fopenmp -emit-llvm -S test.c -o test.ll
IR代码如下：
; ModuleID = 'test.bc'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"
%ident_t = type { i32, i32, i32, i32, i8* }
@.str = private unnamed_addr constant [35 x i8] c"Hello from thread %d, nthreads %d\0A\00", align 1
@.str.1 = private unnamed_addr constant [23 x i8] c";unknown;unknown;0;0;;\00", align 1
@0 = private unnamed_addr constant %ident_t { i32 0, i32 2, i32 0, i32 0, i8* getelementptr inbounds ([23 x i8], [23 x i8]* @.str.1, i32 0, i32 0) }, align 8
; Function Attrs: nounwind uwtable
define i32 @main() #0 {
call void (%ident_t*, i32, void (i32*, i32*, ...)*, ...) @__kmpc_fork_call(%ident_t* @0, i32 0, void (i32*, i32*, ...)* bitcast (void (i32*, i32*)* @.omp_outlined. to void (i32*, i32*, ...)*))
ret i32 0
}
; Function Attrs: nounwind uwtable
define internal void @.omp_outlined.(i32* noalias %.global_tid., i32* noalias %.bound_tid.) #0 {
%1 = alloca i32*, align 8
%2 = alloca i32*, align 8
store i32* %.global_tid., i32** %1, align 8
store i32* %.bound_tid., i32** %2, align 8
%3 = call i32 @omp_get_thread_num()
%4 = call i32 @omp_get_num_threads()
%5 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([35 x i8], [35 x i8]* @.str, i32 0, i32 0), i32 %3, i32 %4)
ret void
}
declare i32 @printf(i8*, ...) #1
declare i32 @omp_get_thread_num() #1
declare i32 @omp_get_num_threads() #1
declare void @__kmpc_fork_call(%ident_t*, i32, void (i32*, i32*, ...)*, ...)
attributes #0 = { nounwind uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
!llvm.ident = !{!0}
!0 = !{!"clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)"}
其中mpc_fork_call是OpenMP定义的运行时函数，用来生成线程池，每个线程将执行函数omp_outlined中的代码，该函数的代码就是对应并行域中的代码。
