The LEA optimization pass visits each basic block of a given machine
function. In each basic block, for each pair of LEAs that differ only
in their displacement fields, we replace all uses of the second LEA
with the first LEA while adjusting the displacement.
Now, without this patch, after all the replacements are made, the
following assert triggers:
assert(MRI->use_empty(LastVReg) &&
"The LEA's def register must have no uses");The replacement loop uses:
for (MachineOperand &MO :
llvm::make_early_inc_range(MRI->use_operands(LastVReg))) {which is equivalent to:
for (auto UI = MRI->use_begin(LastVReg), UE = MRI->use_end();
UI != UE;) {
MachineOperand &MO = *UI++; // <-- Look!That is, immediately after the post increment, make_early_inc_range
already has the iterator for the next iteration in its mind.
The problem is that in one iteration of the loop, we could replace two
uses in a debug instruction like:
DBG_VALUE_LIST !"r", !DIExpression(DW_OP_LLVM_arg, 0), %0:gr64, %0:gr64, ...
So, the iterator for the next iteration becomes invalid. We end up
traversing a garbage use list from that point on. In turn, we don't
get to visit remaining uses.
The patch fixes the problem by switching to a "draining" while loop:
while (!MRI->use_empty(LastVReg)) {
MachineOperand &MO = *MRI->use_begin(LastVReg);
MachineInstr &MI = *MO.getParent();The credit goes to Simon Pilgrim for reducing the test case.