clang - LLVM 如何通过运行时库调用转换 OpenMP 多线程代码？

Question

当我研究LLVM OpenMP Runtime Library document时，我发现有一个关于工作共享的例子:

extern float foo( void );
int main () {
    int i;
    float r = 0.0;
    #pragma omp parallel for schedule(dynamic) reduction(+:r)
    for ( i = 0; i < 10; i ++ ) {
        r += foo();
    }
}

然后它显示转换后的代码，如下所示:

extern float foo( void );
int main () {
    static int zero = 0;
    auto int gtid;
    auto float r = 0.0;
    __kmpc_begin( & loc3, 0 );
    // The gtid is not actually required in this example so could be omitted;
    // We show its initialization here because it is often required for calls into
    // the runtime and should be locally cached like this.
    gtid = __kmpc_global thread num( & loc3 );
    __kmpc_fork call( & loc7, 1, main_7_parallel_3, & r );
    __kmpc_end( & loc0 );
    return 0;
}

struct main_10_reduction_t_5 { float r_10_rpr; };

static kmp_critical_name lck = { 0 };
static ident_t loc10; // loc10.flags should contain KMP_IDENT_ATOMIC_REDUCE bit set
                      // if compiler has generated an atomic reduction.
void main_7_parallel_3( int *gtid, int *btid, float *r_7_shp ) {
    auto int i_7_pr;
    auto int lower, upper, liter, incr;
    auto struct main_10_reduction_t_5 reduce;
    reduce.r_10_rpr = 0.F;
    liter = 0;
    __kmpc_dispatch_init_4( & loc7,*gtid, 35, 0, 9, 1, 1 );
    while ( __kmpc_dispatch_next_4( & loc7, *gtid, & liter, & lower, & upper, & incr
      ) ) {
        for( i_7_pr = lower; upper >= i_7_pr; i_7_pr ++ )
          reduce.r_10_rpr += foo();
    }
    switch( __kmpc_reduce_nowait( & loc10, *gtid, 1, 4, & reduce, main_10_reduce_5, &lck ) ) {
        case 1:
           *r_7_shp += reduce.r_10_rpr;
           __kmpc_end_reduce_nowait( & loc10, *gtid, & lck );
           break;
        case 2:
           __kmpc_atomic_float4_add( & loc10, *gtid, r_7_shp, reduce.r_10_rpr );
           break;
        default:;
    }
}

我花了很多时间寻找 OpenMP 如何转换上面的代码，但仍然找不到像示例那样显示结果的方法，以及它如何在 OpenMP 中工作。

所以，这是我困惑了很长时间的问题:有没有办法像示例一样输出文件或直接显示结果？

Answer 1

您可以检查 LLVM IR(请参阅 https://llvm.org/docs/LangRef.html)。

例如:

clang -fopenmp -O2 -emit-llvm -S -o - example.c 

将以下内容打印到标准输出:

[...]
; Function Attrs: nounwind uwtable
define dso_local i32 @main() local_unnamed_addr #0 {
entry:
  %i = alloca i32, align 4
  %r = alloca float, align 4
  %0 = bitcast i32* %i to i8*
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %0) #4
  %1 = bitcast float* %r to i8*
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %1) #4
  store float 0.000000e+00, float* %r, align 4, !tbaa !2
  call void (%struct.ident_t*, i32, void (i32*, i32*, ...)*, ...) @__kmpc_fork_call(%struct.ident_t* nonnull @0, i32 2, void (i32*, i32*, ...)* bitcast (void (i32*, i32*, i32*, float*)* @.omp_outlined. to void (i32*, i32*, ...)*), i32* nonnull %i, float* nonnull %r) #4
  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %1) #4
  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %0) #4
  ret i32 0
}
[...]

; Function Attrs: norecurse nounwind uwtable
define internal void @.omp_outlined.(i32* noalias nocapture readonly %.global_tid., i32* noalias nocapture readnone %.bound_tid., i32* nocapture readnone dereferenceable(4) %i, float* nocapture dereferenceable(4) %r) #2 {
entry:
  %.omp.lb = alloca i32, align 4
  %.omp.ub = alloca i32, align 4
  %.omp.stride = alloca i32, align 4
  %.omp.is_last = alloca i32, align 4
  %r1 = alloca float, align 4
  %.omp.reduction.red_list = alloca [1 x i8*], align 8
  %0 = bitcast i32* %.omp.lb to i8*
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %0) #4
  store i32 0, i32* %.omp.lb, align 4, !tbaa !6
  %1 = bitcast i32* %.omp.ub to i8*
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %1) #4
  store i32 9, i32* %.omp.ub, align 4, !tbaa !6
  %2 = bitcast i32* %.omp.stride to i8*
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %2) #4
  store i32 1, i32* %.omp.stride, align 4, !tbaa !6
  %3 = bitcast i32* %.omp.is_last to i8*
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %3) #4
  store i32 0, i32* %.omp.is_last, align 4, !tbaa !6
  %4 = bitcast float* %r1 to i8*
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %4) #4
  store float 0.000000e+00, float* %r1, align 4, !tbaa !2
  %5 = load i32, i32* %.global_tid., align 4, !tbaa !6
  tail call void @__kmpc_dispatch_init_4(%struct.ident_t* nonnull @0, i32 %5, i32 35, i32 0, i32 9, i32 1, i32 1) #4
  %6 = call i32 @__kmpc_dispatch_next_4(%struct.ident_t* nonnull @0, i32 %5, i32* nonnull %.omp.is_last, i32* nonnull %.omp.lb, i32* nonnull %.omp.ub, i32* nonnull %.omp.stride) #4
  %tobool14 = icmp eq i32 %6, 0
  br i1 %tobool14, label %omp.dispatch.end, label %omp.dispatch.body

omp.dispatch.cond.loopexit:                       ; preds = %omp.inner.for.body, %omp.dispatch.body
  %7 = call i32 @__kmpc_dispatch_next_4(%struct.ident_t* nonnull @0, i32 %5, i32* nonnull %.omp.is_last, i32* nonnull %.omp.lb, i32* nonnull %.omp.ub, i32* nonnull %.omp.stride) #4
  %tobool = icmp eq i32 %7, 0
  br i1 %tobool, label %omp.dispatch.end, label %omp.dispatch.body

omp.dispatch.body:                                ; preds = %entry, %omp.dispatch.cond.loopexit
  %8 = load i32, i32* %.omp.lb, align 4, !tbaa !6
  %9 = load i32, i32* %.omp.ub, align 4, !tbaa !6, !llvm.mem.parallel_loop_access !8
  %cmp12 = icmp sgt i32 %8, %9
  br i1 %cmp12, label %omp.dispatch.cond.loopexit, label %omp.inner.for.body

omp.inner.for.body:                               ; preds = %omp.dispatch.body, %omp.inner.for.body
  %.omp.iv.013 = phi i32 [ %add4, %omp.inner.for.body ], [ %8, %omp.dispatch.body ]
  %call = call float @foo() #4, !llvm.mem.parallel_loop_access !8
  %10 = load float, float* %r1, align 4, !tbaa !2, !llvm.mem.parallel_loop_access !8
  %add3 = fadd float %call, %10
  store float %add3, float* %r1, align 4, !tbaa !2, !llvm.mem.parallel_loop_access !8
  %add4 = add nsw i32 %.omp.iv.013, 1
  %11 = load i32, i32* %.omp.ub, align 4, !tbaa !6, !llvm.mem.parallel_loop_access !8
  %cmp = icmp slt i32 %.omp.iv.013, %11
  br i1 %cmp, label %omp.inner.for.body, label %omp.dispatch.cond.loopexit, !llvm.loop !8

omp.dispatch.end:                                 ; preds = %omp.dispatch.cond.loopexit, %entry
  %12 = bitcast [1 x i8*]* %.omp.reduction.red_list to float**
  store float* %r1, float** %12, align 8
  %13 = bitcast [1 x i8*]* %.omp.reduction.red_list to i8*
  %14 = call i32 @__kmpc_reduce_nowait(%struct.ident_t* nonnull @1, i32 %5, i32 1, i64 8, i8* nonnull %13, void (i8*, i8*)* nonnull @.omp.reduction.reduction_func, [8 x i32]* nonnull @.gomp_critical_user_.reduction.var) #4
  switch i32 %14, label %.omp.reduction.default [
    i32 1, label %.omp.reduction.case1
    i32 2, label %.omp.reduction.case2
  ]

.omp.reduction.case1:                             ; preds = %omp.dispatch.end
  %15 = load float, float* %r, align 4, !tbaa !2
  %16 = load float, float* %r1, align 4, !tbaa !2
  %add5 = fadd float %15, %16
  store float %add5, float* %r, align 4, !tbaa !2
  call void @__kmpc_end_reduce_nowait(%struct.ident_t* nonnull @1, i32 %5, [8 x i32]* nonnull @.gomp_critical_user_.reduction.var) #4
  br label %.omp.reduction.default

.omp.reduction.case2:                             ; preds = %omp.dispatch.end
  %17 = bitcast float* %r to i32*
  %atomic-load = load atomic i32, i32* %17 monotonic, align 4, !tbaa !2
  %18 = load float, float* %r1, align 4, !tbaa !2
  br label %atomic_cont

atomic_cont:                                      ; preds = %atomic_cont, %.omp.reduction.case2
  %19 = phi i32 [ %atomic-load, %.omp.reduction.case2 ], [ %23, %atomic_cont ]
  %20 = bitcast i32 %19 to float
  %add7 = fadd float %18, %20
  %21 = bitcast float %add7 to i32
  %22 = cmpxchg i32* %17, i32 %19, i32 %21 monotonic monotonic
  %23 = extractvalue { i32, i1 } %22, 0
  %24 = extractvalue { i32, i1 } %22, 1
  br i1 %24, label %.omp.reduction.default, label %atomic_cont

.omp.reduction.default:                           ; preds = %atomic_cont, %.omp.reduction.case1, %omp.dispatch.end
  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %4) #4
  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %3) #4
  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %2) #4
  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %1) #4
  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %0) #4
  ret void
}
[...]