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Scott Manley via llvm-dev

2019-May-02 21:14 UTC

[llvm-dev] llvm is illegally vectorizing with a recurrence on skylake

Hi -- I have found a bug in an HPC code where llvm is vectorizing a loop on
Skylake that has an obvious recurrence. I derived a small test case based
on the original benchmark below:

/*****************************************************************/
static void  __attribute__ ((always_inline)) one(
  const int *restrict in, const int *const end,
  const unsigned shift, int *const restrict index,
  int *const restrict out)
{
  do {
    int a_idx = *in>>shift;
    int b_idx = index[a_idx];
    out[b_idx] = *in;                // <-- reccurence as index[a_idx] can
be the
    index[a_idx]++;                 //      same and incremented within the
vector
  } while(++in!=end);              //     which leads to incorrect results
}

#ifndef NO_TWO
static void  __attribute__ ((noinline)) two(
  const int *restrict in, const int *const end,
  const unsigned shift, int *const restrict index,
  int *const restrict out)
{
  do out[index[(*in>>shift)]++]=*in; while(++in!=end);
}
#endif

void parent(
  int digits, int n, int *restrict work, int * restrict idx,
  int *restrict shift, int **restrict indicies)
{
  int *in = work;
  int *dst = work+n;
//  int *indicies[1024];
//  int shift[1024];
  int d;
  for(d=1;d!=digits-1;++d) {
    int *t;
    one(in,in+n,shift[d],indicies[d],dst);
    t=in,in=dst,dst=t;
  }
#ifndef NO_TWO
  two(in,in+n,shift[d],indicies[d],idx);
#endif
}
/*****************************************************************/

clang -S -O2 -Rpass=loop-vectorize small.c  -march=skylake-avx512
small.c:6:3: remark: vectorized loop (vectorization width: 16, interleaved
count: 1) [-Rpass=loop-vectorize]
  do {
  ^

I believe the problem to be a issue with dependency information getting
destroyed because if you remove the two() function (or compile one() on its
own, or prevent inlining of one()), it correctly prevents vectorization.

clang -S -O2 -Rpass=loop-vectorize -Rpass-missed=loop-vectorize small.c
-march=skylake-avx512 -DNO_TWO
small.c:6:3: remark: loop not vectorized [-Rpass-missed=loop-vectorize]
  do {

I did trace it down to possibly being something within
DepChecker->areDepsSafe() as it returns true for the incorrect case.

Thanks,

Scott
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Finkel, Hal J. via llvm-dev

2019-May-02 23:53 UTC

head link

[llvm-dev] llvm is illegally vectorizing with a recurrence on skylake

Hi, Scott,

Thanks for reporting this problem. We should get a bug filed on this issue at
bugs.llvm.org. If you're not able to do this, please let us know, and
someone else can take care of it.

 -Hal

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Scott
Manley via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Thursday, May 2, 2019 4:14 PM
To: llvm-dev
Subject: [llvm-dev] llvm is illegally vectorizing with a recurrence on skylake

Hi -- I have found a bug in an HPC code where llvm is vectorizing a loop on
Skylake that has an obvious recurrence. I derived a small test case based on the
original benchmark below:

/*****************************************************************/
static void  __attribute__ ((always_inline)) one(
  const int *restrict in, const int *const end,
  const unsigned shift, int *const restrict index,
  int *const restrict out)
{
  do {
    int a_idx = *in>>shift;
    int b_idx = index[a_idx];
    out[b_idx] = *in;                // <-- reccurence as index[a_idx] can be
the
    index[a_idx]++;                 //      same and incremented within the
vector
  } while(++in!=end);              //     which leads to incorrect results
}

#ifndef NO_TWO
static void  __attribute__ ((noinline)) two(
  const int *restrict in, const int *const end,
  const unsigned shift, int *const restrict index,
  int *const restrict out)
{
  do out[index[(*in>>shift)]++]=*in; while(++in!=end);
}
#endif

void parent(
  int digits, int n, int *restrict work, int * restrict idx,
  int *restrict shift, int **restrict indicies)
{
  int *in = work;
  int *dst = work+n;
//  int *indicies[1024];
//  int shift[1024];
  int d;
  for(d=1;d!=digits-1;++d) {
    int *t;
    one(in,in+n,shift[d],indicies[d],dst);
    t=in,in=dst,dst=t;
  }
#ifndef NO_TWO
  two(in,in+n,shift[d],indicies[d],idx);
#endif
}
/*****************************************************************/

clang -S -O2 -Rpass=loop-vectorize small.c  -march=skylake-avx512
small.c:6:3: remark: vectorized loop (vectorization width: 16, interleaved
count: 1) [-Rpass=loop-vectorize]
  do {
  ^

I believe the problem to be a issue with dependency information getting
destroyed because if you remove the two() function (or compile one() on its own,
or prevent inlining of one()), it correctly prevents vectorization.

clang -S -O2 -Rpass=loop-vectorize -Rpass-missed=loop-vectorize small.c 
-march=skylake-avx512 -DNO_TWO
small.c:6:3: remark: loop not vectorized [-Rpass-missed=loop-vectorize]
  do {

I did trace it down to possibly being something within
DepChecker->areDepsSafe() as it returns true for the incorrect case.

Thanks,

Scott
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Scott Manley via llvm-dev

2019-May-02 23:55 UTC

head link

[llvm-dev] llvm is illegally vectorizing with a recurrence on skylake

I can file a bug, no problem. I've just seen folks start on the list first.

Cheers,

Scott


On Thu, May 2, 2019, 6:53 PM Finkel, Hal J. <hfinkel at anl.gov> wrote:
> Hi, Scott,
>
> Thanks for reporting this problem. We should get a bug filed on this issue
> at bugs.llvm.org. If you're not able to do this, please let us know,
and
> someone else can take care of it.
>
>  -Hal
>
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> ------------------------------
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Scott
> Manley via llvm-dev <llvm-dev at lists.llvm.org>
> *Sent:* Thursday, May 2, 2019 4:14 PM
> *To:* llvm-dev
> *Subject:* [llvm-dev] llvm is illegally vectorizing with a recurrence on
> skylake
>
> Hi -- I have found a bug in an HPC code where llvm is vectorizing a loop
> on Skylake that has an obvious recurrence. I derived a small test case
> based on the original benchmark below:
>
> /*****************************************************************/
> static void  __attribute__ ((always_inline)) one(
>   const int *restrict in, const int *const end,
>   const unsigned shift, int *const restrict index,
>   int *const restrict out)
> {
>   do {
>     int a_idx = *in>>shift;
>     int b_idx = index[a_idx];
>     out[b_idx] = *in;                // <-- reccurence as index[a_idx]
can
> be the
>     index[a_idx]++;                 //      same and incremented within
> the vector
>   } while(++in!=end);              //     which leads to incorrect results
> }
>
> #ifndef NO_TWO
> static void  __attribute__ ((noinline)) two(
>   const int *restrict in, const int *const end,
>   const unsigned shift, int *const restrict index,
>   int *const restrict out)
> {
>   do out[index[(*in>>shift)]++]=*in; while(++in!=end);
> }
> #endif
>
> void parent(
>   int digits, int n, int *restrict work, int * restrict idx,
>   int *restrict shift, int **restrict indicies)
> {
>   int *in = work;
>   int *dst = work+n;
> //  int *indicies[1024];
> //  int shift[1024];
>   int d;
>   for(d=1;d!=digits-1;++d) {
>     int *t;
>     one(in,in+n,shift[d],indicies[d],dst);
>     t=in,in=dst,dst=t;
>   }
> #ifndef NO_TWO
>   two(in,in+n,shift[d],indicies[d],idx);
> #endif
> }
> /*****************************************************************/
>
> clang -S -O2 -Rpass=loop-vectorize small.c  -march=skylake-avx512
> small.c:6:3: remark: vectorized loop (vectorization width: 16, interleaved
> count: 1) [-Rpass=loop-vectorize]
>   do {
>   ^
>
> I believe the problem to be a issue with dependency information getting
> destroyed because if you remove the two() function (or compile one() on its
> own, or prevent inlining of one()), it correctly prevents vectorization.
>
> clang -S -O2 -Rpass=loop-vectorize -Rpass-missed=loop-vectorize small.c
> -march=skylake-avx512 -DNO_TWO
> small.c:6:3: remark: loop not vectorized [-Rpass-missed=loop-vectorize]
>   do {
>
> I did trace it down to possibly being something within
> DepChecker->areDepsSafe() as it returns true for the incorrect case.
>
> Thanks,
>
> Scott
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