[PATCH] D37740: [SelectionDAG] Pick correct frame index in LowerArguments

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope created this revision.
Herald added subscribers: igorb, aprantl, nemanjai.

SelectionDAGISel::LowerArguments is associating arguments
with frame indices (FuncInfo->setArgumentFrameIndex). That
information is later on used by EmitFuncArgumentDbgValue to
create DBG_VALUE instructions that denotes that a variable
can be found on the stack.

I discovered that for our (big endian) out-of-tree target
the association created by SelectionDAGISel::LowerArguments
sometimes is wrong. I've seen this happen when a 64-bit value
is passed on the stack. The argument will occupy two stack
slots (frame index X, and frame index X+1). The fault is
that a call to setArgumentFrameIndex is associating the
64-bit argument with frame index X+1. The effect is that the
debug information (DBG_VALUE) will point at the least significant
part of the arguement on the stack. When printing the
argument in a debugger I will get the wrong value.

I managed to create a test case for PowerPC that seems to
show the same kind of problem.

The bugfix will look at the datalayout, taking endianess into
account when examining a BUILD_PAIR node, assuming that the
least significant part is in the first operand of the BUILD_PAIR.
For big endian targets we should use the frame index from
the second operand, as the most significant part will be stored
at the lower address (using the highest frame index).

I also added an assert that we always pick the operand associated
with the highest frame index in this particular piece of code.


https://reviews.llvm.org/D37740

Files:
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  test/CodeGen/PowerPC/debuginfo-stackarg.ll

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