lower: fix a bug causing undefined variables when applying fuse

[rohany]

Fixes #355.

This commit fixes a bug where the fuse transformation would not generate
necessary locator variables when applied to iteration over two dense
variables.

Additionally, this commit adds a test for when a dense iteration results
in a transposition of a tensor.

[rohany]

cc @stephenchouca, this is another attempt at #360. Instead of generating parent locators at a particular point, I instead generate locators on the way down through the iteration graph for variables that must be recovered.

[stephenchouca]

Looks good as far as I can tell! Thanks!

[Infinoid]

Hi,

This patch causes a test failure for me.

Configure with -DCUDA=ON. Then run the scheduling_eval.spmvGPU test:

bin/taco-test --gtest_filter=scheduling_eval.spmvGPU

With revision 468ad7f (merge this PR):

% bin/taco-test --gtest_filter=scheduling_eval.spmvGPU
Note: Google Test filter = scheduling_eval.spmvGPU
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from scheduling_eval
[ RUN      ] scheduling_eval.spmvGPU
zsh: segmentation fault (core dumped)  bin/taco-test --gtest_filter=scheduling_eval.spmvGPU

With revision 468ad7f^ (master branch prior to merge):

% bin/taco-test --gtest_filter=scheduling_eval.spmvGPU
Note: Google Test filter = scheduling_eval.spmvGPU
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from scheduling_eval
[ RUN      ] scheduling_eval.spmvGPU
[       OK ] scheduling_eval.spmvGPU (8408 ms)
[----------] 1 test from scheduling_eval (8408 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (8408 ms total)
[  PASSED  ] 1 test.

When I trigger it in gdb:

Thread 1 "taco-test" received signal SIGSEGV, Segmentation fault.
taco::LowererImpl::declLocatePosVars (this=0x55555665ddf0, locators=...)
    at /home/infinoid/workspace/taco/git/src/lower/lowerer_impl.cpp:2288
2288	        taco_iassert(isValue(locate.getResults()[1], true));
(gdb) p locateIterator.hasLocate()
$1 = false
(gdb) p locate
$2 = {content = std::shared_ptr<taco::ModeFunction::Content> (empty) = {get() = 0x0}}

Apparently the ModeFormat claims that it can't locate, and locateIterator.locate() returns a NULL pointer. The call to locate.getResults() then segfaults.

I also noticed that the coords argument to locateIterator.locate() is a vector of 2 values in this case, it contains fposA twice. Normally coords only contains a single Var.

(gdb) p coords
$1 = std::vector of length 2, capacity 2 = {{<taco::ir::IRHandle> = {<taco::util::IntrusivePtr<taco::ir::IRNode const>> = {
        _vptr.IntrusivePtr = 0x555555a0e778 <vtable for taco::ir::Expr+16>, 
        ptr = 0x55555665c8e0}, <No data fields>}, <No data fields>}, 
  {<taco::ir::IRHandle> = {<taco::util::IntrusivePtr<taco::ir::IRNode const>> = {
        _vptr.IntrusivePtr = 0x555555a0e778 <vtable for taco::ir::Expr+16>, 
        ptr = 0x55555665c8e0}, <No data fields>}, <No data fields>}}

Github CI didn't catch this failure, because it doesn't run the GPU tests.

[rohany]

Can you open an issue about it? I or someone else can take a look at it later