RFR: 8290910: Wrong memory state is picked in SuperWord::co_locate_pack() [v2]

[Fei Gao]

> After [JDK-8283091](https://bugs.openjdk.org/browse/JDK-8283091), the loop below can be vectorized partially.
> Statement 1 can be vectorized but statement 2 can't.
> 
> // int[] iArr; long[] lArrFld; int i1,i2;
> for (i1 = 6; i1 < 227; i1++) {
>   iArr[i1] += lArrFld[i1]++; // statement 1
>   iArr[i1 + 1] -= (i2++); // statement 2
> }
> 
> 
> But we got incorrect results because the vector packs of iArr are
> scheduled incorrectly like:
> 
> ...
> load_vector XMM1,[R8 + #16 + R11 << #2]
> movl    RDI, [R8 + #20 + R11 << #2] # int
> load_vector XMM2,[R9 + #8 + R11 << #3]
> subl    RDI, R11    # int
> vpaddq  XMM3,XMM2,XMM0  ! add packedL
> store_vector [R9 + #8 + R11 << #3],XMM3
> vector_cast_l2x  XMM2,XMM2  !
> vpaddd  XMM1,XMM2,XMM1  ! add packedI
> addl    RDI, #228   # int
> movl    [R8 + #20 + R11 << #2], RDI # int
> movl    RBX, [R8 + #24 + R11 << #2] # int
> subl    RBX, R11    # int
> addl    RBX, #227   # int
> movl    [R8 + #24 + R11 << #2], RBX # int
> ...
> movl    RBX, [R8 + #40 + R11 << #2] # int
> subl    RBX, R11    # int
> addl    RBX, #223   # int
> movl    [R8 + #40 + R11 << #2], RBX # int
> movl    RDI, [R8 + #44 + R11 << #2] # int
> subl    RDI, R11    # int
> addl    RDI, #222   # int
> movl    [R8 + #44 + R11 << #2], RDI # int
> store_vector [R8 + #16 + R11 << #2],XMM1
> ...
> 
> simplified as:
> 
> load_vector iArr in statement 1
> unvectorized loads/stores in statement 2
> store_vector iArr in statement 1
> 
> We cannot pick the memory state from the first load for LoadI pack
> here, as the LoadI vector operation must load the new values in memory
> after iArr writes `iArr[i1 + 1] - (i2++)` to `iArr[i1 + 1]`(statement 2).
> We must take the memory state of the last load where we have assigned
> new values `iArr[i1 + 1] - (i2++)` to the iArr array.
> 
> In [JDK-8240281](https://bugs.openjdk.org/browse/JDK-8240281), we picked the memory state of the first load[1]. Different
> from the scenario in [JDK-8240281](https://bugs.openjdk.org/browse/JDK-8240281), the store, which is dependent on an
> earlier load here, is in a pack to be scheduled and the LoadI pack
> depends on the last_mem. As designed[2], to schedule the StoreI pack,
> all memory operations in another single pack should be moved in the same
> direction. We know that the store in the pack depends on one of loads in
> the LoadI pack, so the LoadI pack should be scheduled before the StoreI
> pack. And the LoadI pack depends on the last_mem, so the last_mem must
> be scheduled before the LoadI pack and also before the store pack.
> Therefore, we need to take the memory state of the last load for the
> LoadI pack here.
> 
> To fix it, the pack adds additional checks while picking the memory state
> of the first load. When the store locates in a pack and the load pack
> relies on the last_mem, we shouldn't choose the memory state of the
> first load but choose the memory state of the last load.
> 
> [1]https://github.com/openjdk/jdk/blob/0ae834105740f7cf73fe96be22e0f564ad29b18d/src/hotspot/share/opto/superword.cpp#L2380
> [2]https://github.com/openjdk/jdk/blob/0ae834105740f7cf73fe96be22e0f564ad29b18d/src/hotspot/share/opto/superword.cpp#L2232

Fei Gao has updated the pull request incrementally with one additional commit since the last revision:

  Code style change: add one space
  
  Change-Id: I2794060ac0f9dbe006e32f202111ee08f09d96a1

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9898/files
  - new: https://git.openjdk.org/jdk/pull/9898/files/77277fed..01d64113

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9898&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9898&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9898.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9898/head:pull/9898

PR: https://git.openjdk.org/jdk/pull/9898