LLVM结构数组迭代

Question

使用LLVM编译此代码时:

struct bar {
    int int1;
    int int2;
    char char1;
    char char2;
    char char3;
};


struct foo {
    struct bar array[16];
};


int func(struct foo *f, int num) {

    for(int i = 0; i < num; i++){
        f->array[i].int1 = 1;
        f->array[i].int2 = 2;
        f->array[i].char1 = 'a';
        f->array[i].char2 = 'b';        
        f->array[i].char3 = 'c';        
    }
    return num;
}

由于某种原因,编译器决定以奇怪的方式迭代这个数组.首先,它在结构的中间或末尾选择一个看似任意的点,然后使用相对于任意点的immediates存储适当的值.

我发现从这个IR代码中选择了任意点:

  %scevgep = getelementptr %struct.foo* %f, i32 0, i32 0, i32 0, i32 4

其中4是char3的偏移量.

在这个例子中,int1,int2,char1,char2的存储将具有负的immediates,char3将立即为0.

似乎使用结构条的不同排列,您可以得到不同的偏移量,但始终在结构体的内部或末尾.

例如,将struct bar更改为:

struct bar {
    char char1;
    char char2;
    char char3;
    int int1;
    int int2;
};

将产生以下IR线:

  %scevgep = getelementptr %struct.foo* %f, i32 0, i32 0, i32 0, i32 3

这意味着char1,char2和char3的存储将具有负的immediates,int1将立即为0,而int2将具有正的立即数.

为什么它相对于结构基础以外的点进行迭代？

Answer 1

最近构建的 Clang 不会生成getelementptr您所描述的指令。它使用正常的索引。它所做的最奇怪的事情是生成一个循环体展开两次的版本：

target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

%struct.foo = type { [16 x %struct.bar] }
%struct.bar = type { i32, i32, i8, i8, i8 }

define i32 @func(%struct.foo* nocapture %f, i32 %num) {
entry:
  %cmp25 = icmp sgt i32 %num, 0
  br i1 %cmp25, label %for.body.preheader, label %for.cond.cleanup

for.body.preheader:                               ; preds = %entry
  %xtraiter = and i32 %num, 1
  %0 = icmp eq i32 %num, 1
  br i1 %0, label %for.cond.cleanup.loopexit.unr-lcssa, label %for.body.preheader.new

for.body.preheader.new:                           ; preds = %for.body.preheader
  %unroll_iter = sub i32 %num, %xtraiter
  br label %for.body

for.cond.cleanup.loopexit.unr-lcssa.loopexit:     ; preds = %for.body
  %indvars.iv.next.1.lcssa = phi i64 [ %indvars.iv.next.1, %for.body ]
  br label %for.cond.cleanup.loopexit.unr-lcssa

for.cond.cleanup.loopexit.unr-lcssa:              ; preds = %for.cond.cleanup.loopexit.unr-lcssa.loopexit, %for.body.preheader
  %indvars.iv.unr = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.next.1.lcssa, %for.cond.cleanup.loopexit.unr-lcssa.loopexit ]
  %lcmp.mod = icmp eq i32 %xtraiter, 0
  br i1 %lcmp.mod, label %for.cond.cleanup.loopexit, label %for.body.epil.preheader

for.body.epil.preheader:                          ; preds = %for.cond.cleanup.loopexit.unr-lcssa
  br label %for.body.epil

for.body.epil:                                    ; preds = %for.body.epil.preheader
  %int1.epil = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.unr, i32 0
  store i32 1, i32* %int1.epil, align 4, !tbaa !1
  %int2.epil = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.unr, i32 1
  store i32 2, i32* %int2.epil, align 4, !tbaa !6
  %char1.epil = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.unr, i32 2
  store i8 97, i8* %char1.epil, align 4, !tbaa !7
  %char2.epil = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.unr, i32 3
  store i8 98, i8* %char2.epil, align 1, !tbaa !8
  %char3.epil = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.unr, i32 4
  store i8 99, i8* %char3.epil, align 2, !tbaa !9
  br label %for.cond.cleanup.loopexit.epilog-lcssa

for.cond.cleanup.loopexit.epilog-lcssa:           ; preds = %for.body.epil
  br label %for.cond.cleanup.loopexit

for.cond.cleanup.loopexit:                        ; preds = %for.cond.cleanup.loopexit.unr-lcssa, %for.cond.cleanup.loopexit.epilog-lcssa
  br label %for.cond.cleanup

for.cond.cleanup:                                 ; preds = %for.cond.cleanup.loopexit, %entry
  ret i32 %num

for.body:                                         ; preds = %for.body, %for.body.preheader.new
  %indvars.iv = phi i64 [ 0, %for.body.preheader.new ], [ %indvars.iv.next.1, %for.body ]
  %niter = phi i32 [ %unroll_iter, %for.body.preheader.new ], [ %niter.nsub.1, %for.body ]
  %int1 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv, i32 0
  store i32 1, i32* %int1, align 4, !tbaa !1
  %int2 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv, i32 1
  store i32 2, i32* %int2, align 4, !tbaa !6
  %char1 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv, i32 2
  store i8 97, i8* %char1, align 4, !tbaa !7
  %char2 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv, i32 3
  store i8 98, i8* %char2, align 1, !tbaa !8
  %char3 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv, i32 4
  store i8 99, i8* %char3, align 2, !tbaa !9
  %indvars.iv.next = or i64 %indvars.iv, 1
  %int1.1 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.next, i32 0
  store i32 1, i32* %int1.1, align 4, !tbaa !1
  %int2.1 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.next, i32 1
  store i32 2, i32* %int2.1, align 4, !tbaa !6
  %char1.1 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.next, i32 2
  store i8 97, i8* %char1.1, align 4, !tbaa !7
  %char2.1 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.next, i32 3
  store i8 98, i8* %char2.1, align 1, !tbaa !8
  %char3.1 = getelementptr inbounds %struct.foo, %struct.foo* %f, i64 0, i32 0, i64 %indvars.iv.next, i32 4
  store i8 99, i8* %char3.1, align 2, !tbaa !9
  %indvars.iv.next.1 = add nsw i64 %indvars.iv, 2
  %niter.nsub.1 = add i32 %niter, -2
  %niter.ncmp.1 = icmp eq i32 %niter.nsub.1, 0
  br i1 %niter.ncmp.1, label %for.cond.cleanup.loopexit.unr-lcssa.loopexit, label %for.body
}

如果您使用重现您所看到的 IR 的步骤来更新您的问题，我很乐意解释为什么 LLVM 生成它，但我不想根据指令名称进行猜测。