Unlike what I claimed in my previous commit. The caching is
actually not NFC on PHIs.
When we put a big enough max depth, we end up simulating loops.
The cache is effectively cutting the simulation short and we
get less information as a result.
E.g.,
```
v0 = G_CONSTANT i8 0xC0
jump
v1 = G_PHI i8 v0, v2
v2 = G_LSHR i8 v1, 1
```
Let say we want the known bits of v1.
- With cache:
Set v1 cache to we know nothing
v1 is v0 & v2
v0 gives us 0xC0
v2 gives us known bits of v1 >> 1
v1 is in the cache
=> v1 is 0, thus v2 is 0x80
Finally v1 is v0 & v2 => 0x80
- Without cache and enough depth to do two iteration of the loop:
v1 is v0 & v2
v0 gives us 0xC0
v2 gives us known bits of v1 >> 1
v1 is v0 & v2
v0 is 0xC0
v2 is v1 >> 1
Reach the max depth for v1...
unwinding
v1 is know nothing
v2 is 0x80
v0 is 0xC0
v1 is 0x80
v2 is 0xC0
v0 is 0xC0
v1 is 0xC0
Thus now v1 is 0xC0 instead of 0x80.
I've added a unittest demonstrating that.
NFC
This patch adds a cache that is valid only for the duration of a call
to getKnownBits. With such short lived cache we avoid all the problems
of cache invalidation while still getting the benefits of reusing
the information we already computed.
This cache is useful whenever an instruction occurs more than once
in a chain of computation.
E.g.,
v0 = G_ADD v1, v2
v3 = G_ADD v0, v1
Previously we would compute the known bits for:
v1, v2, v0, then v1 again and finally v3.
With the patch, now we won't have to recompute v1 again.
NFC
When analyzing PHIs, we gather the known bits for every operand and
merge them together to get the known bits of the result of the PHI.
It is not unusual that merging the information leads to know nothing
on the result (e.g., phi a: i8 3, b: i8 unknown, ..., after looking at the
second argument we know we will know nothing on the result), thus, as
soon as we reach that state, stop analyzing the following operand (i.e.,
on the previous example, we won't process anything after looking at `b`).
This improves compile time in particular with PHIs with a large number
of operands.
NFC.
Summary:
This was a very odd API, where you had to pass a flag into a zext
function to say whether the extended bits really were zero or not. All
callers passed in a literal true or false.
I think it's much clearer to make the function name reflect the
operation being performed on the value we're tracking (rather than on
the KnownBits Zero and One fields), so zext means the value is being
zero extended and new function anyext means the value is being extended
with unknown bits.
NFC.
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D74482
One of the exit criteria of computeKnownBits is whether we reach the max
recursive call depth. Before this patch we would check that the
depth is exactly equal to max depth to exit.
Depth may get bigger than max depth if it gets passed to a different
GISelKnownBits object.
This may happen when say a generic part uses a GISelKnownBits object
with some max depth, but then we hit TL.computeKnownBitsForTargetInstr
which creates a new GISelKnownBits object with a different and smaller
depth. In that situation, when we hit the max depth check for the first
time in the target specific GISelKnownBits object, depth may already
be bigger than the current max depth. Hence we would continue to compute
the known bits, until we ran through the full depth of the chain of
computation or ran out of stack space.
For instance, let say we have
GISelKnownBits Info(/*MaxDepth*/ = 10);
Info.getKnownBits(Foo)
// 9 recursive calls to computeKnownBitsImpl.
// Then we hit a target specific instruction.
// The target specific GISelKnownBits does this:
GISelKnownBits TargetSpecificInfo(/*MaxDepth*/ = 6)
TargetSpecificInfo.computeKnownBitsImpl() // <-- next max depth checks would
// always return false.
This commit does not have any test case, none of the in-tree targets
use computeKnownBitsForTargetInstr.
Summary:
This is mostly NFC. computeForAddSub may give more precise results in
some cases, but that doesn't seem to affect any existing GlobalISel
tests.
Subscribers: rovka, hiraditya, volkan, Petar.Avramovic, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D73431
Teach the GISelKnowBits analysis how to deal with PHI operations.
PHIs are essentially COPYs happening on edges, so we can just reuse
the code for COPY.
This is NFC COPY-wise has we leave Depth untouched when calling
computeKnownBitsImpl for COPYs, like it was before this patch.
Increasing Depth is however required for PHIs as they may loop back to
themselves and we would end up in an infinite loop if we were not
increasing Depth.
Differential Revision: https://reviews.llvm.org/D73317
Summary:
G_GEP is rather poorly named. It's a simple pointer+scalar addition and
doesn't support any of the complexities of getelementptr. I therefore
propose that we rename it. There's a G_PTR_MASK so let's follow that
convention and go with G_PTR_ADD
Reviewers: volkan, aditya_nandakumar, bogner, rovka, arsenm
Subscribers: sdardis, jvesely, wdng, nhaehnle, hiraditya, jrtc27, atanasyan, arphaman, Petar.Avramovic, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D69734
Summary:
It seems we missed that the target hook can't query the known-bits for the
inputs to a target instruction. Fix that oversight
Reviewers: aditya_nandakumar
Subscribers: rovka, hiraditya, volkan, Petar.Avramovic, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D67380
llvm-svn: 373264
Now that we look through copies, it's possible to visit registers that
have a register class constraint but not a type constraint. Avoid looking
through copies when this occurs as the SrcReg won't be able to determine
it's bit width or any known bits.
Along the same lines, if the initial query is on a register that doesn't
have a type constraint then the result is a default-constructed KnownBits,
that is, a 1-bit fully-unknown value.
llvm-svn: 371116
https://reviews.llvm.org/D66039
We were using getIndexSize instead of getIndexSizeInBits().
Added test case for G_PTRTOINT and G_INTTOPTR.
llvm-svn: 368618
https://reviews.llvm.org/D65698
This adds a KnownBits analysis pass for GISel. This was done as a
pass (compared to static functions) so that we can add other features
such as caching queries(within a pass and across passes) in the future.
This patch only adds the basic pass boiler plate, and implements a lazy
non caching knownbits implementation (ported from SelectionDAG). I've
also hooked up the AArch64PreLegalizerCombiner pass to use this - there
should be no compile time regression as the analysis is lazy.
llvm-svn: 368065
Quentin Colombet