Jyotirmoy Bhattacharya via llvm-dev
2017-May-10 07:16 UTC
[llvm-dev] autovectorization of outer loop
I have the following C++ code that evaluates a Chebyshev polynomial using
Clenshaw's algorithm
void cheby_eval(double *coeffs,int n,double *xs,double *ys,int m)
{
#pragma omp simd
for (int i=0;i<m;i++){
double x = xs[i];
double u0=0,u1=0,u2=0;
for (int k=n;k>=0;k--){
u2 = u1;
u1 = u0;
u0 = 2*x*u1-u2+coeffs[k];
}
ys[i] = 0.5*(coeffs[0]+u0-u2);
}
}
I'm hoping for an autovectorization of the outer loop so that the inner
loop operates on vectors.
When compiled with
clang++ -O3 -march=haswell -Rpass-analysis=loop-vectorize -S chebyshev.cc
using clang++ 3.8.1-23, no vectorization happens and I get the message
chebyshev.cc:19:18: remark: loop not vectorized: cannot identify array
bounds
[-Rpass-analysis=loop-vectorize]
ys[i] = 0.5*(coeffs[0]+u0-u2);
^
chebyshev.cc:21:1: remark: loop not vectorized: value that could not be
identified as reduction is used outside the loop
[-Rpass-analysis=loop-vectorize]
On the same code icc vectorizes the outer loop as expected.
I was wondering if there are small ways in which I can change my code to
help LLVM's autovectorizer to succeed. I would also appreciate any pointers
to documentation or LLVM source that can help me better understand how
autovectorization of outer loops works.
Regards,
Jyotirmoy Bhattacharya
PS. The interesting part of icc's assembler output is
..B1.4: # Preds ..B1.8 ..B1.3
xorl %r15d, %r15d #14.5
xorl %ebx, %ebx #14.21
testq %rsi, %rsi #14.21
vmovupd (%rdx,%r9,8), %ymm3 #12.16
vxorpd %ymm5, %ymm5, %ymm5 #13.14
vmovdqa %ymm1, %ymm4 #13.19
vmovdqa %ymm1, %ymm2 #13.24
jl ..B1.8 # Prob 2% #14.21
..B1.5: # Preds ..B1.4
vaddpd %ymm3, %ymm3, %ymm3 #17.14
..B1.6: # Preds ..B1.6 ..B1.5
vmovapd %ymm4, %ymm2 #20.3
incq %r15 #14.5
vmovapd %ymm5, %ymm4 #20.3
vfmsub213pd %ymm2, %ymm3, %ymm5 #17.19
vbroadcastsd (%r11,%rbx,8), %ymm6 #17.22
decq %rbx
vaddpd %ymm5, %ymm6, %ymm5 #17.22
cmpq %r10, %r15 #14.5
jb ..B1.6 # Prob 82% #14.5
..B1.8: # Preds ..B1.6 ..B1.4
vbroadcastsd (%rdi), %ymm3 #19.18
vaddpd %ymm3, %ymm5, %ymm4 #19.28
vsubpd %ymm2, %ymm4, %ymm2 #19.31
vmulpd %ymm2, %ymm0, %ymm5 #19.31
vmovupd %ymm5, (%rcx,%r9,8) #19.5
addq $4, %r9 #11.3
cmpq %r8, %r9 #11.3
jb ..B1.4 # Prob 82% #11
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> help me better understand how autovectorization of outer loops works.LLVM’s loop vectorizer currently handles innermost loops only.> I'm hoping for an autovectorization of the outer loop so that the inner loop operates on vectors.We share that hope and are working to achieve it: http://lists.llvm.org/pipermail/llvm-dev/2016-September/105057.html, but it will take some time. See https://reviews.llvm.org/D28975 and https://reviews.llvm.org/D32871. Thanks for the use-case.> I was wondering if there are small ways in which I can change my code to help LLVM's autovectorizer to succeed.If a doubly-nested loop can be interchanged such that the inner loop becomes vectorizable, it may help. Ayal. From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Jyotirmoy Bhattacharya via llvm-dev Sent: Wednesday, May 10, 2017 10:16 To: llvm-dev at lists.llvm.org Subject: [llvm-dev] autovectorization of outer loop I have the following C++ code that evaluates a Chebyshev polynomial using Clenshaw's algorithm void cheby_eval(double *coeffs,int n,double *xs,double *ys,int m) { #pragma omp simd for (int i=0;i<m;i++){ double x = xs[i]; double u0=0,u1=0,u2=0; for (int k=n;k>=0;k--){ u2 = u1; u1 = u0; u0 = 2*x*u1-u2+coeffs[k]; } ys[i] = 0.5*(coeffs[0]+u0-u2); } } I'm hoping for an autovectorization of the outer loop so that the inner loop operates on vectors. When compiled with clang++ -O3 -march=haswell -Rpass-analysis=loop-vectorize -S chebyshev.cc using clang++ 3.8.1-23, no vectorization happens and I get the message chebyshev.cc:19:18: remark: loop not vectorized: cannot identify array bounds [-Rpass-analysis=loop-vectorize] ys[i] = 0.5*(coeffs[0]+u0-u2); ^ chebyshev.cc:21:1: remark: loop not vectorized: value that could not be identified as reduction is used outside the loop [-Rpass-analysis=loop-vectorize] On the same code icc vectorizes the outer loop as expected. I was wondering if there are small ways in which I can change my code to help LLVM's autovectorizer to succeed. I would also appreciate any pointers to documentation or LLVM source that can help me better understand how autovectorization of outer loops works. Regards, Jyotirmoy Bhattacharya PS. The interesting part of icc's assembler output is ..B1.4: # Preds ..B1.8 ..B1.3 xorl %r15d, %r15d #14.5 xorl %ebx, %ebx #14.21 testq %rsi, %rsi #14.21 vmovupd (%rdx,%r9,8), %ymm3 #12.16 vxorpd %ymm5, %ymm5, %ymm5 #13.14 vmovdqa %ymm1, %ymm4 #13.19 vmovdqa %ymm1, %ymm2 #13.24 jl ..B1.8 # Prob 2% #14.21 ..B1.5: # Preds ..B1.4 vaddpd %ymm3, %ymm3, %ymm3 #17.14 ..B1.6: # Preds ..B1.6 ..B1.5 vmovapd %ymm4, %ymm2 #20.3 incq %r15 #14.5 vmovapd %ymm5, %ymm4 #20.3 vfmsub213pd %ymm2, %ymm3, %ymm5 #17.19 vbroadcastsd (%r11,%rbx,8), %ymm6 #17.22 decq %rbx vaddpd %ymm5, %ymm6, %ymm5 #17.22 cmpq %r10, %r15 #14.5 jb ..B1.6 # Prob 82% #14.5 ..B1.8: # Preds ..B1.6 ..B1.4 vbroadcastsd (%rdi), %ymm3 #19.18 vaddpd %ymm3, %ymm5, %ymm4 #19.28 vsubpd %ymm2, %ymm4, %ymm2 #19.31 vmulpd %ymm2, %ymm0, %ymm5 #19.31 vmovupd %ymm5, (%rcx,%r9,8) #19.5 addq $4, %r9 #11.3 cmpq %r8, %r9 #11.3 jb ..B1.4 # Prob 82% #11 --------------------------------------------------------------------- Intel Israel (74) Limited This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170510/35b94455/attachment.html>