[cfe-users] Memory accesses to struct variables in LLVM IR

[David Blaikie via cfe-users]

On Thu, Feb 11, 2016 at 7:25 AM, Simona Simona via cfe-users <
cfe-users at lists.llvm.org> wrote:

> Hi,
>
> I'm using clang 3.4 to generate the bitcode of a C source file.
> The source file is the following:
>
> typedef struct __attribute__ ((__packed__)) { float x, y; } myType;
> myType make_float2(float x, float y) { myType f = { x, y }; return f; }
>
> int main(int argc, char* argv[])
> {
>         myType myVar[5];
>
>         for(int i=0;i<5;i++)
>                 myVar[i] = make_float2(i,i);
>
>         return(myVar[1].x);
> }
>
> The bitcode is generated using the following command:
> clang -c -emit-llvm -O0 -fno-vectorize -fno-slp-vectorize
> -fno-lax-vector-conversions main.c -o main.bc
>
> target triple = "x86_64-unknown-linux-gnu"
>
> %struct.myType = type <{ float, float }>
>
> ; Function Attrs: nounwind uwtable
> define <2 x float> @_Z11make_float2ff(float %x, float %y) #0 {
> entry:
>   %retval = alloca %struct.myType, align 1
>   %x1 = getelementptr inbounds %struct.myType* %retval, i32 0, i32 0
>   store float %x, float* %x1, align 1
>   %y2 = getelementptr inbounds %struct.myType* %retval, i32 0, i32 1
>   store float %y, float* %y2, align 1
>   %0 = bitcast %struct.myType* %retval to <2 x float>*
>   %1 = load <2 x float>* %0, align 1
>   ret <2 x float> %1
> }
>
> ; Function Attrs: nounwind uwtable
> define i32 @main(i32 %argc, i8** %argv) #0 {
> entry:
>   %myVar = alloca [100 x %struct.myType], align 16
>

Looks like your IR corresponds to an array of length 100, not 5 as in your
source, but that's not too important


>  * %ref.tmp = alloca %struct.myType, align 1*
>   br label %for.cond
>
> for.cond:                                         ; preds = %for.inc,
> %entry
>   %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
>   %cmp = icmp slt i32 %i.0, 5
>   br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>   %idxprom = sext i32 %i.0 to i64
>   %arrayidx = getelementptr inbounds [100 x %struct.myType]* %myVar, i32
> 0, i64 %idxprom
>   %conv = sitofp i32 %i.0 to float
>   %conv1 = sitofp i32 %i.0 to float
>  * %call = call <2 x float> @_Z11make_float2ff(float %conv, float %conv1)*
> *  %0 = bitcast %struct.myType* %ref.tmp to <2 x float>**
> *  store <2 x float> %call, <2 x float>* %0, align 1*
>   %1 = bitcast %struct.myType* %arrayidx to i8*
>   %2 = bitcast %struct.myType* %ref.tmp to i8*
>   call void @llvm.memcpy.p0i8.p0i8.i64(i8* %1, i8* %2, i64 8, i32 1, i1
> false)
>

Here is the store ^ into your array (%1 is the destination, a bitcast of
%arrayidx, which is the pointer into your array at index %idxprom, which is
%i.0, etc) using the memcpy intrinsic, rather than a store instruction.


>   br label %for.inc
>
> for.inc:                                          ; preds = %for.body
>   %inc = add nsw i32 %i.0, 1
>   br label %for.cond
>
> for.end:                                          ; preds = %for.cond
>   %arrayidx2 = getelementptr inbounds [100 x %struct.myType]* %myVar, i32
> 0, i64 1
>   %x = getelementptr inbounds %struct.myType* %arrayidx2, i32 0, i32 0
>   %3 = load float* %x, align 1
>   %conv3 = fptosi float %3 to i32
>   ret i32 %conv3
> }
>
> Looking at the C source code there should be 5 store instructions
> corresponding to the 5 assignments of myVar[0], myVar[1], myVar[2],
> myVar[3] and myVar[4].
> When I look at the bitcode however, I see 5 instances of *store <2 x
> float> %call, <2 x float>* %0, align 1 *which correspond to 5 stores at
> the same address
> of %0 (which is actually %ref.tmp defined as *%ref.tmp = alloca
> %struct.myType, align 1*).
>
> I would appreciate it if anyone could let me know how the 5 memory
> accesses at the 5 *different* memory addresses are implemented in the
> bitcode.
>
> Thanks,
> Simona
>
>
> _______________________________________________
> cfe-users mailing list
> cfe-users at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-users/attachments/20160211/ab4b446d/attachment.html>