[PATCH] D34347: [PowerPC] fix potential verification error on __tls_get_addr

[Nemanja Ivanovic via Phabricator via llvm-commits]

nemanjai added a comment.

I think we should add a test case for this. Of course, since these are pseudo instructions we're dealing with, it would probably have to be an mir test.



================
Comment at: lib/Target/PowerPC/PPCTLSDynamicCall.cpp:61
         MachineInstr &MI = *I;
+        if (MI.getOpcode() == PPC::ADJCALLSTACKDOWN) {
+          NeedFence = false;
----------------
If the `ADJCALLSTACKDOWN` instruction is guaranteed to occur before any of the "TLS" pseudos in a basic block, that should be mentioned in a comment here. If on the other hand it isn't guaranteed to happen, we can form adjacent `ADJCALLSTACKDOWN/ADJCALLSTACKUP` blocks.

I don't imagine that's a problem, but it is somewhat inconsistent here...
What I mean is that if this loop encounters an `ADJCALLSTACKDOWN` which is followed by an `ADJCALLSTACKUP`, we will still not emit the instructions for the subsequent TLS instructions. I am not sure if that is a problem or not.

So the crux of what I'm saying here is this:
```
...
ADJCALLSTACKDOWN
...
ADJCALLSTACKUP
...
ADDItlsgdLADDR
```
Will not add the adjustments. Whereas this will (for all of the TLS instructions):
```
...
ADDItlsgdLADDR
...
ADDItlsgdLADDR
```
And this will add the adjustments only for the TLS instructions that appear before the stack adjustments that were already there:
```
...
ADDItlsgdLADDR
...
ADJCALLSTACKDOWN
...
ADJCALLSTACKUP
...
ADDItlsgdLADDR
```


https://reviews.llvm.org/D34347