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[LegalizeTypes] Bugfixes for big-endian targets when handling BITCASTs

Authored by uabelho on Dec 9 2019, 11:09 PM.

Description

[LegalizeTypes] Bugfixes for big-endian targets when handling BITCASTs

Summary:
This fixes PR44135.

The special case when we promote a bitcast from a vector to an int
needs special handling when we are on a big-endian target.

Prior to this fix, for the added vec_to_int we see the following in the
SelectionDAG printouts

Type-legalized selection DAG: %bb.1 'foo:bb.1'
SelectionDAG has 9 nodes:

t0: ch = EntryToken
      t2: v8i16,ch = CopyFromReg t0, Register:v8i16 %0
    t17: v4i32 = bitcast t2
  t23: i32 = extract_vector_elt t17, Constant:i32<3>
t8: ch,glue = CopyToReg t0, Register:i32 $r0, t23
t9: ch = ARMISD::RET_FLAG t8, Register:i32 $r0, t8:1

and I think here the extract_vector_elt is wrong and extracts the value
from the wrong index.

The program program should return the 32 bits made up of the elements at
index 4 and 5 in the vec6 array, but with

t23: i32 = extract_vector_elt t17, Constant:i32<3>

as far as I can tell, we will extract values that originally didn't even
exist in the vec6 vectore.

If we would instead extract the element at index 2 we would get the wanted
values.

With this fix we insert a right shift after the bitcast in
DAGTypeLegalizer::PromoteIntRes_BITCAST which then gives us

Type-legalized selection DAG: %bb.1 'vec_to_int:bb.1'
SelectionDAG has 9 nodes:

t0: ch = EntryToken
      t2: v8i16,ch = CopyFromReg t0, Register:v8i16 %0
    t23: v4i32 = bitcast t2
  t27: i32 = extract_vector_elt t23, Constant:i32<2>
t8: ch,glue = CopyToReg t0, Register:i32 $r0, t27
t9: ch = ARMISD::RET_FLAG t8, Register:i32 $r0, t8:1

So now we get

t27: i32 = extract_vector_elt t23, Constant:i32<2>

which is what we want.

Similarly, the new int_to_vec testcase exposes a bug where we cast the other
direction. Then we instead need to add a left shift before the bitcast on
big-endian targets for the bits in the input integer to end up at the exptected
place in the vector.

Reviewers: bogner, spatel, craig.topper, t.p.northover, dmgreen, efriedma, SjoerdMeijer, samparker

Reviewed By: efriedma

Subscribers: eli.friedman, bjope, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70942

Details

Committed
uabelhoDec 10 2019, 2:22 AM
Reviewer
efriedma
Differential Revision
D70942: [LegalizeTypes] Bugfixes for big-endian targets when handling BITCASTs
Parents
rG4d280d3ac06a: Add testcases exposing PR44135
Branches
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Tags
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