llvm-6502/lib/Target/Mips/MSA.txt

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Code Generation Notes for MSA
=============================
Intrinsics are lowered to SelectionDAG nodes where possible in order to enable
optimisation, reduce the size of the ISel matcher, and reduce repetition in
the implementation. In a small number of cases, this can cause different
(semantically equivalent) instructions to be used in place of the requested
instruction, even when no optimisation has taken place.
Instructions
============
This section describes any quirks of instruction selection for MSA. For
example, two instructions might be equally valid for some given IR and one is
chosen in preference to the other.
bclri.b:
It is not possible to emit bclri.b since andi.b covers exactly the
same cases. andi.b should use fractionally less power than bclri.b in
most hardware implementations so it is used in preference to bclri.b.
vshf.w:
It is not possible to emit vshf.w when the shuffle description is
constant since shf.w covers exactly the same cases. shf.w is used
instead. It is also impossible for the shuffle description to be
unknown at compile-time due to the definition of shufflevector in
LLVM IR.
vshf.[bhwd]
When the shuffle description describes a splat operation, splat.[bhwd]
instructions will be selected instead of vshf.[bhwd]. Unlike the ilv*,
and pck* instructions, this is matched from MipsISD::VSHF instead of
a special-case MipsISD node.
ilvl.d, pckev.d:
It is not possible to emit ilvl.d, or pckev.d since ilvev.d covers the
same shuffle. ilvev.d will be emitted instead.
ilvr.d, ilvod.d, pckod.d:
It is not possible to emit ilvr.d, or pckod.d since ilvod.d covers the
same shuffle. ilvod.d will be emitted instead.
splat.[bhwd]
The intrinsic will work as expected. However, unlike other intrinsics
it lowers directly to MipsISD::VSHF instead of using common IR.
splati.w:
It is not possible to emit splati.w since shf.w covers the same cases.
shf.w will be emitted instead.
copy_s.w:
On MIPS32, the copy_u.d intrinsic will emit this instruction instead of
copy_u.w. This is semantically equivalent since the general-purpose
register file is 32-bits wide.
binsri.[bhwd], binsli.[bhwd]:
These two operations are equivalent to each other with the operands
swapped and condition inverted. The compiler may use either one as
appropriate.
Furthermore, the compiler may use bsel.[bhwd] for some masks that do
not survive the legalization process (this is a bug and will be fixed).
bmnz.v, bmz.v, bsel.v:
These three operations differ only in the operand that is tied to the
[mips] BSEL's and BINS[RL] operands are reversed compared to the vselect node used in the pattern. Summary: Correct the match patterns and the lowerings that made the CodeGen tests pass despite the mistakes. The original testcase that discovered the problem was SingleSource/UnitTests/SignlessType/factor.c in test-suite. During review, we also found that some of the existing CodeGen tests were incorrect and fixed them: * bitwise.ll: In bsel_v16i8 the IfSet/IfClear were reversed because bsel and bmnz have different operand orders and the test didn't correctly account for this. bmnz goes 'IfClear, IfSet, CondMask', while bsel goes 'CondMask, IfClear, IfSet'. * vec.ll: In the cases where a bsel is emitted as a bmnz (they are the same operation with a different input tied to the result) the operands were in the wrong order. * compare.ll and compare_float.ll: The bsel operand order was correct for a greater-than comparison, but a greater-than comparison instruction doesn't exist. Lowering this operation inverts the condition so the IfSet/IfClear need to be swapped to match. The differences between BSEL, BMNZ, and BMZ and how they map to/from vselect are rather confusing. I've therefore added a note to MSA.txt to explain this in a single place in addition to the comments that explain each case. Reviewers: matheusalmeida, jacksprat Reviewed By: matheusalmeida Differential Revision: http://llvm-reviews.chandlerc.com/D3028 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203657 91177308-0d34-0410-b5e6-96231b3b80d8
2014-03-12 11:54:00 +00:00
result and the order of the operands.
It is (currently) not possible to emit bmz.v, or bsel.v since bmnz.v is
the same operation and will be emitted instead.
In future, the compiler may choose between these three instructions
according to register allocation.
[mips] BSEL's and BINS[RL] operands are reversed compared to the vselect node used in the pattern. Summary: Correct the match patterns and the lowerings that made the CodeGen tests pass despite the mistakes. The original testcase that discovered the problem was SingleSource/UnitTests/SignlessType/factor.c in test-suite. During review, we also found that some of the existing CodeGen tests were incorrect and fixed them: * bitwise.ll: In bsel_v16i8 the IfSet/IfClear were reversed because bsel and bmnz have different operand orders and the test didn't correctly account for this. bmnz goes 'IfClear, IfSet, CondMask', while bsel goes 'CondMask, IfClear, IfSet'. * vec.ll: In the cases where a bsel is emitted as a bmnz (they are the same operation with a different input tied to the result) the operands were in the wrong order. * compare.ll and compare_float.ll: The bsel operand order was correct for a greater-than comparison, but a greater-than comparison instruction doesn't exist. Lowering this operation inverts the condition so the IfSet/IfClear need to be swapped to match. The differences between BSEL, BMNZ, and BMZ and how they map to/from vselect are rather confusing. I've therefore added a note to MSA.txt to explain this in a single place in addition to the comments that explain each case. Reviewers: matheusalmeida, jacksprat Reviewed By: matheusalmeida Differential Revision: http://llvm-reviews.chandlerc.com/D3028 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203657 91177308-0d34-0410-b5e6-96231b3b80d8
2014-03-12 11:54:00 +00:00
These three operations can be very confusing so here is a mapping
between the instructions and the vselect node in one place:
bmz.v wd, ws, wt/i8 -> (vselect wt/i8, wd, ws)
bmnz.v wd, ws, wt/i8 -> (vselect wt/i8, ws, wd)
bsel.v wd, ws, wt/i8 -> (vselect wd, wt/i8, ws)
bmnzi.b, bmzi.b:
Like their non-immediate counterparts, bmnzi.v and bmzi.v are the same
operation with the operands swapped. bmnzi.v will (currently) be emitted
for both cases.
bseli.v:
Unlike the non-immediate versions, bseli.v is distinguishable from
bmnzi.b and bmzi.b and can be emitted.