[LLVMdev] Converting a i32 pointer to a vector of i32 ( C array to LLVM vector)
Matthieu Dubet
maattdd at gmail.com
Wed Oct 16 19:39:57 PDT 2013
Hi,
So I've tried the Loop vectorizer and the SLP vectorizer (LLVM 3.3) on this
code : (which is assigning 5 to each element of the array "%b")
; ModuleID = 'res.ll'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
; Function Attrs: nounwind
define void @loop_ptr_19121568([10 x i32*]* nocapture %params_vec) #0 {
entry:
%0 = bitcast [10 x i32*]* %params_vec to double**
%temp_1 = load double** %0, align 8
%1 = getelementptr [10 x i32*]* %params_vec, i64 0, i64 1
%2 = load i32** %1, align 8
%temp_2 = bitcast i32* %2 to double*
%3 = getelementptr [10 x i32*]* %params_vec, i64 0, i64 3
%4 = load i32** %3, align 8
%b = bitcast i32* %4 to double*
%5 = getelementptr [10 x i32*]* %params_vec, i64 0, i64 4
%6 = load i32** %5, align 8
%d = bitcast i32* %6 to double*
%7 = getelementptr [10 x i32*]* %params_vec, i64 0, i64 6
%8 = load i32** %7, align 8
%temp_0 = bitcast i32* %8 to double*
%9 = getelementptr [10 x i32*]* %params_vec, i64 0, i64 7
%10 = load i32** %9, align 8
%temp_4 = bitcast i32* %10 to i1*
%11 = getelementptr [10 x i32*]* %params_vec, i64 0, i64 8
%12 = load i32** %11, align 8
%temp_3 = bitcast i32* %12 to double*
%13 = getelementptr [10 x i32*]* %params_vec, i64 0, i64 9
%14 = load i32** %13, align 8
%i = bitcast i32* %14 to double*
store double 1.000000e+00, double* %temp_0, align 8
%15 = load double* %temp_1, align 8
store double %15, double* %temp_2, align 8
%16 = load double* %d, align 8
%17 = fmul double %16, %16
store double %17, double* %temp_3, align 8
%.pre = load double* %temp_0, align 8
%cmp_le1 = fcmp ole double %.pre, %17
store i1 %cmp_le1, i1* %temp_4, align 1
br i1 %cmp_le1, label %"i = temp_0", label %end_fun
"i = temp_0": ; preds = %entry, %"i =
temp_0"
%18 = load double* %temp_0, align 8
store double %18, double* %i, align 8
%19 = fptoui double %18 to i32
%20 = add i32 %19, -1
%21 = sext i32 %20 to i64
%22 = getelementptr double* %b, i64 %21
store double 5.000000e+00, double* %22, align 8
%23 = load double* %temp_0, align 8
%24 = load double* %temp_2, align 8
%25 = fadd double %23, %24
store double %25, double* %temp_0, align 8
%.pre1 = load double* %temp_3, align 8
%cmp_le = fcmp ole double %25, %.pre1
store i1 %cmp_le, i1* %temp_4, align 1
br i1 %cmp_le, label %"i = temp_0", label %end_fun
end_fun: ; preds = %"i = temp_0",
%entry
ret void
}
attributes #0 = { nounwind }
-------------------------------------------------------------------------------------------------------
The loop vectorizer find the loop, but I don't exactly get the trouble with
the loop exit count ..
LV: Checking a loop in "loop_ptr_19121568"
LV: Found a loop: i = temp_0
LV: SCEV could not compute the loop exit count.
LV: Not vectorizing.
The SLP vectorizer debug :
SLP: Vectorizing a list of length = 2.
SLP: Cost of pair:1 Cost of extract:1.
SLP: Vectorizing a list of length = 2.
SLP: Cost of pair:1 Cost of extract:1.
SLP: Found 4 stores to vectorize.
SLP: Vectorizing a list of length = 2.
SLP: Cost of pair:1 Cost of extract:1.
SLP: Vectorizing a list of length = 2.
SLP: Cost of pair:1 Cost of extract:1.
SLP: Found 4 stores to vectorize.
Thanks,
Matthieu
On Wed, Oct 16, 2013 at 8:59 PM, Nadav Rotem <nrotem at apple.com> wrote:
> Both the SLP vectorizer and the Loop vectorizer support vectorizing
> pointers. The attached code looks like a candidate for the SLP-vectorizer.
> Can you run the SLP-vectorizer with the flag -mllvm -debug-only=SLP and
> attach the log ? I think that we are missing the pattern for the roots of
> the tree.
>
> Thanks,
> Nadav
>
>
> On Oct 16, 2013, at 5:28 PM, Tom Stellard <tom at stellard.net> wrote:
>
> On Wed, Oct 16, 2013 at 11:14:06AM -0400, Matthieu Dubet wrote:
>
> Hi,
>
> Thank you for the information,
>
> So I'm now keeping the array as a pointer (i32*) but the vectorizer doesn't
> vectorize it .
>
> I've pasted the function code before and after optimization (and the list
> of optimization that I have activated) in this Gist :
> https://gist.github.com/maattd/7008683
>
> Some "weird" fact of my LLVM code :
>
> * all variables (even the one used for the loop condition) are pointers to
> memory allocated from the C world and passed to the LLVM functions as an
> argument
> * even with "opt->add(new llvm::DataLayout(*ee->getDataLayout())) ;" in the
> code, the module->dump() doesn't output neither data layout, nor triple
> target
>
> Both those points might confuse the vectorizer ?
>
>
> On Fri, Oct 11, 2013 at 1:40 PM, Renato Golin <renato.golin at linaro.org
> >wrote:
>
> On 11 October 2013 18:27, Matthieu Dubet <maattdd at gmail.com> wrote:
>
> How can I tell LLVM to consider this i32* as an <10 x i32> (and thus get
> the performance improvements thanks to SIMD ..etc..) ?
>
>
> Hi Matthieu,
>
> You shouldn't need to do anything, the vectorizer should spot that for
> you, if the machine you're compiling to has support for vector
> instructions. Any kind of vector operations that you may want to hard-code
> will make it not work on anything other than the intrinsics/inline asm
> you're using, which is not a good idea.
>
>
> Which part of the vectorizer is responsible for doing pointer->vector
> transformations?
>
> -Tom
>
> If your code didn't get vectorized, it's possible that it is not clear
> enough that that pointer is being iterated in a way that it's easy for the
> vectorizer to spot, so maybe you need to make it clearer, and that depends
> on the code in question. If you could share the code (or a similar example)
> with the list, people could help you spot the pattern and make it
> vectorize.
>
> cheers,
> --renato
>
>
>
>
>
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