[LLVMdev] Why is the loop vectorizer not working on my function?
Frank Winter
fwinter at jlab.org
Sat Oct 26 12:16:27 PDT 2013
Hi Hal!
I am using the 'x86_64' target. Below the complete module dump and here
the command line:
opt -march=x64-64 -loop-vectorize -debug-only=loop-vectorize -S test.ll
Frank
; ModuleID = 'test.ll'
target datalayout =
"e-p:64:64:64-S128-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f16:16:16-f32:32:32-f64:64:64-f128:128:128-v64:64:64-v128:12
8:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-unknown-linux-elf"
define void @bar([8 x i8]* %arg_ptr) {
entrypoint:
%0 = bitcast [8 x i8]* %arg_ptr to i32*
%1 = load i32* %0
%2 = getelementptr [8 x i8]* %arg_ptr, i32 1
%3 = bitcast [8 x i8]* %2 to i32*
%4 = load i32* %3
%5 = getelementptr [8 x i8]* %arg_ptr, i32 2
%6 = bitcast [8 x i8]* %5 to float**
%7 = load float** %6
%8 = getelementptr [8 x i8]* %arg_ptr, i32 3
%9 = bitcast [8 x i8]* %8 to float**
%10 = load float** %9
%11 = getelementptr [8 x i8]* %arg_ptr, i32 4
%12 = bitcast [8 x i8]* %11 to float**
%13 = load float** %12
br label %L0
L0: ; preds = %L0, %entrypoint
%14 = phi i32 [ %21, %L0 ], [ %1, %entrypoint ]
%15 = getelementptr float* %10, i32 %14
%16 = load float* %15
%17 = getelementptr float* %13, i32 %14
%18 = load float* %17
%19 = fmul float %18, %16
%20 = getelementptr float* %7, i32 %14
store float %19, float* %20
%21 = add nsw i32 %14, 1
%22 = icmp sge i32 %21, %4
br i1 %22, label %L1, label %L0
L1: ; preds = %L0
ret void
}
On 26/10/13 15:08, Hal Finkel wrote:
> ----- Original Message -----
>> Hi Arnold,
>>
>> adding '-debug-only=loop-vectorize' to the command gives:
>>
>> LV: Checking a loop in "bar"
>> LV: Found a loop: L0
>> LV: Found an induction variable.
>> LV: Found an unidentified write ptr: %7 = load float** %6
>> LV: Found an unidentified read ptr: %10 = load float** %9
>> LV: Found an unidentified read ptr: %13 = load float** %12
>> LV: We need to do 2 pointer comparisons.
>> LV: We can't vectorize because we can't find the array bounds.
>> LV: Can't vectorize due to memory conflicts
>> LV: Not vectorizing.
>>
>> It can't find the loop bounds if we use the overflow version of add.
>> That's a good point. I should mark this addition to not overflow.
>>
>> When using the non-overflow version I get:
>>
>> LV: Checking a loop in "bar"
>> LV: Found a loop: L0
>> LV: Found an induction variable.
>> LV: Found an unidentified write ptr: %7 = load float** %6
>> LV: Found an unidentified read ptr: %10 = load float** %9
>> LV: Found an unidentified read ptr: %13 = load float** %12
>> LV: Found a runtime check ptr: %20 = getelementptr float* %7, i32
>> %14
>> LV: Found a runtime check ptr: %15 = getelementptr float* %10, i32
>> %14
>> LV: Found a runtime check ptr: %17 = getelementptr float* %13, i32
>> %14
>> LV: We need to do 2 pointer comparisons.
>> LV: We can perform a memory runtime check if needed.
>> LV: We need a runtime memory check.
>> LV: We can vectorize this loop (with a runtime bound check)!
>> LV: Found trip count: 0
>> LV: The Widest type: 32 bits.
>> LV: The Widest register is: 32 bits.
>> LV: Found an estimated cost of 0 for VF 1 For instruction: %14 =
>> phi
>> i32 [ %21, %L0 ], [ %1, %entrypoint ]
>> LV: Found an estimated cost of 0 for VF 1 For instruction: %15 =
>> getelementptr float* %10, i32 %14
>> LV: Found an estimated cost of 1 for VF 1 For instruction: %16 =
>> load
>> float* %15
>> LV: Found an estimated cost of 0 for VF 1 For instruction: %17 =
>> getelementptr float* %13, i32 %14
>> LV: Found an estimated cost of 1 for VF 1 For instruction: %18 =
>> load
>> float* %17
>> LV: Found an estimated cost of 1 for VF 1 For instruction: %19 =
>> fmul
>> float %18, %16
>> LV: Found an estimated cost of 0 for VF 1 For instruction: %20 =
>> getelementptr float* %7, i32 %14
>> LV: Found an estimated cost of 1 for VF 1 For instruction: store
>> float
>> %19, float* %20
>> LV: Found an estimated cost of 1 for VF 1 For instruction: %21 =
>> add
>> nsw i32 %14, 1
>> LV: Found an estimated cost of 1 for VF 1 For instruction: %22 =
>> icmp
>> sge i32 %21, %4
>> LV: Found an estimated cost of 1 for VF 1 For instruction: br i1
>> %22,
>> label %L1, label %L0
>> LV: Scalar loop costs: 7.
>> LV: Selecting VF = : 1.
>> LV: The target has 8 vector registers
>> LV(REG): Calculating max register usage:
>> LV(REG): At #0 Interval # 0
>> LV(REG): At #1 Interval # 1
>> LV(REG): At #2 Interval # 2
>> LV(REG): At #3 Interval # 2
>> LV(REG): At #4 Interval # 3
>> LV(REG): At #5 Interval # 3
>> LV(REG): At #6 Interval # 2
>> LV(REG): At #8 Interval # 1
>> LV(REG): At #9 Interval # 1
>> LV(REG): Found max usage: 3
>> LV(REG): Found invariant usage: 5
>> LV(REG): LoopSize: 11
>> LV: Vectorization is possible but not beneficial.
>> LV: Found a vectorizable loop (1) in saxpy_real.gvn.mod.ll
>> LV: Unroll Factor is 1
>>
>> It's not beneficial? I didn't expect that. Do you have a descriptive
>> explanation why it's not beneficial?
> It looks like the vectorizer is not picking up a TTI implementation from a target with vector registers (likely, you're just seeing the basic cost model). For what target is this?
>
> -Hal
>
>> Frank
>>
>>
>>
>> On 26/10/13 13:03, Arnold wrote:
>>> Hi Frank,
>>>
>>> Sent from my iPhone
>>>
>>>> On Oct 26, 2013, at 10:03 AM, Frank Winter <fwinter at jlab.org>
>>>> wrote:
>>>>
>>>> My function implements a simple loop:
>>>>
>>>> void bar( int start, int end, float* A, float* B, float* C)
>>>> {
>>>> for (int i=start; i<end;++i)
>>>> A[i] = B[i] * C[i];
>>>> }
>>>>
>>>> This looks pretty much like the standard example. However, I built
>>>> the function
>>>> with the IRBuilder, thus not coming from C and clang. Also I
>>>> changed slightly
>>>> the function's signature:
>>>>
>>>> define void @bar([8 x i8]* %arg_ptr) {
>>>> entrypoint:
>>>> %0 = bitcast [8 x i8]* %arg_ptr to i32*
>>>> %1 = load i32* %0
>>>> %2 = getelementptr [8 x i8]* %arg_ptr, i32 1
>>>> %3 = bitcast [8 x i8]* %2 to i32*
>>>> %4 = load i32* %3
>>>> %5 = getelementptr [8 x i8]* %arg_ptr, i32 2
>>>> %6 = bitcast [8 x i8]* %5 to float**
>>>> %7 = load float** %6
>>>> %8 = getelementptr [8 x i8]* %arg_ptr, i32 3
>>>> %9 = bitcast [8 x i8]* %8 to float**
>>>> %10 = load float** %9
>>>> %11 = getelementptr [8 x i8]* %arg_ptr, i32 4
>>>> %12 = bitcast [8 x i8]* %11 to float**
>>>> %13 = load float** %12
>>>> br label %L0
>>>>
>>>> L0: ; preds = %L0,
>>>> %entrypoint
>>>> %14 = phi i32 [ %21, %L0 ], [ %1, %entrypoint ]
>>>> %15 = getelementptr float* %10, i32 %14
>>>> %16 = load float* %15
>>>> %17 = getelementptr float* %13, i32 %14
>>>> %18 = load float* %17
>>>> %19 = fmul float %18, %16
>>>> %20 = getelementptr float* %7, i32 %14
>>>> store float %19, float* %20
>>>> %21 = add i32 %14, 1
>>> Try
>>> %21 = add nsw i32 %14, 1
>>> instead for no-signed wrapping arithmetic.
>>>
>>> If that is not working please post the output of opt ...
>>> -debug-only=loop-vectorize ...
>>>
>>>
>>>
>>>> %22 = icmp sge i32 %21, %4
>>>> br i1 %22, label %L1, label %L0
>>>>
>>>> L1: ; preds = %L0
>>>> ret void
>>>> }
>>>>
>>>>
>>>> As you can see, I use the phi instruction for the loop index. I
>>>> notice
>>>> that clang prefers stack allocation. So, I am not sure what's the
>>>> problem that the loop vectorizer is not working here.
>>>> I tried many things, like specifying an architecture with vector
>>>> units, enforcing the vector width. No success.
>>>>
>>>> opt -march=x64-64 -loop-vectorize -force-vector-width=8 -S loop.ll
>>>>
>>>> The only explanation I have is the use of the phi instruction. Is
>>>> this
>>>> preventing to vectorize the loop?
>>>>
>>>> Frank
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
More information about the llvm-dev
mailing list