O3都是怪物,这里分析的是CLANG怪物,示例程序遍历数组每个元素然后放大。
void foreach_scale(int arr[],int elem){
for(int i=0;i<elem;i++){
arr[i] += (elem*1024);
}
}
这里删去了用处不大的内容,只保留了关键的LLVM IR。通过分析可以看到,如果循环小于8 LLVM IR会使用vector,vector使用SIMD指令高效进行计算,如果大于8则是普通的for形式。
; Function Attrs: norecurse nounwind
define void @"\01?foreach_scale@@YAXQAHH@Z"(i32* nocapture %arr, i32 %elem) local_unnamed_addr #0 {
entry:
;elem>0则进入循环,否则整个函数结束
%cmp5 = icmp sgt i32 %elem, 0
br i1 %cmp5, label %for.body.lr.ph, label %for.cond.cleanup
for.body.lr.ph:
%mul = shl i32 %elem, 10;
; elem和8进行比较(utl表示unsigned less than)
; elem<8则跳到正常循环%for.body.preheader,否则跳到%vector.ph
%min.iters.check = icmp ult i32 %elem, 8
br i1 %min.iters.check, label %for.body.preheader, label %vector.ph
for.body.preheader:
;phi表示SSA里面的φ函数,详细参见LLVM DOC
%i.06.ph = phi i32 [ 0, %for.body.lr.ph ], [ %n.vec, %middle.block ]
br label %for.body
vector.ph:
%n.vec = and i32 %elem, -8
;首先构造<%mul val val val>,然后shufflevector构造<%mul %mul %mul %mul>
%broadcast.splatinsert9 = insertelement <4 x i32> undef, i32 %mul, i32 0
%broadcast.splat10 = shufflevector <4 x i32> %broadcast.splatinsert9, <4 x i32> undef, <4 x i32> zeroinitializer
;ditto, %broadcast.splatinsert9 == <%mul %mul %mul %mul>
%broadcast.splatinsert11 = insertelement <4 x i32> undef, i32 %mul, i32 0
%broadcast.splat12 = shufflevector <4 x i32> %broadcast.splatinsert11, <4 x i32> undef, <4 x i32> zeroinitializer
br label %vector.body
vector.body:
%index = phi i32 [ 0, %vector.ph ], [ %index.next, %vector.body ]
;从arr指向的内存加载数据
;%0现在表示<arr[0] arr[1] arr[2] arr[3]>
%0 = getelementoptr inbounds i32, i32* %arr, i32 %index
%1 = bitcast i32* %0 to <4 x i32>*
%wide.load = load <4 x i32>, <4 x i32>* %1, align 4, !tbaa !3
;%2表示<arr[4] arr[5] arr[6] arr[7]>
%2 = getelementptr i32, i32* %0, i32 4
%3 = bitcast i32* %2 to <4 x i32>*
%wide.load8 = load <4 x i32>, <4 x i32>* %3, align 4, !tbaa !3
;<arr[0] arr[1] arr[2] arr[3]>与<%mul %mul %mul %mul>相加,得到vector:%4
;<arr[4] arr[5] arr[6] arr[7]>与<%mul %mul %mul %mul>相加,得到vector:%5
%4 = add nsw <4 x i32> %wide.load, %broadcast.splat10
%5 = add nsw <4 x i32> %wide.load8, %broadcast.splat12
;%4,%5写回内存
%6 = bitcast i32* %0 to <4 x i32>*
store <4 x i32> %4, <4 x i32>* %6, align 4, !tbaa !3
%7 = bitcast i32* %2 to <4 x i32>*
store <4 x i32> %5, <4 x i32>* %7, align 4, !tbaa !3
%index.next = add i32 %index, 8
%8 = icmp eq i32 %index.next, %n.vec
br i1 %8, label %middle.block, label %vector.body, !llvm.loop !7
middle.block:
%cmp.n = icmp eq i32 %n.vec, %elem
br i1 %cmp.n, label %for.cond.cleanup, label %for.body.preheader
;函数返回
for.cond.cleanup:
ret void
for.body:
%i.06 = phi i32 [ %inc, %for.body ], [ %i.06.ph, %for.body.preheader ]‘
; arr[i] = arr[i]+ (elem*1024),其中%mul=(elem*1024)
%arrayidx = getelementptr inbounds i32, i32* %arr, i32 %i.06
%9 = load i32, i32* %arrayidx, align 4, !tbaa !3
%add = add nsw i32 %9, %mul
store i32 %add, i32* %arrayidx, align 4, !tbaa !3
; i++
%inc = add nuw nsw i32 %i.06, 1
; 循环条件i<elem判断
%exitcond = icmp eq i32 %inc, %elem
br i1 %exitcond, label %for.cond.cleanup, label %for.body, !llvm.loop !10
}