[PATCH] D136497: [Clang] support for outputs along indirect edges of asm goto

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers added inline comments.


================
Comment at: clang/lib/CodeGen/CGStmt.cpp:2868-2873
+      // If we happen to share the same indirect and default dest, don't re-add
+      // stores. That was done for the default destination in the above call to
+      // EmitAsmStores.
+      llvm::BasicBlock *Succ = CBR->getIndirectDest(i);
+      if (Succ == CBR->getDefaultDest())
+        continue;
----------------
nickdesaulniers wrote:
> void wrote:
> > nickdesaulniers wrote:
> > > nickdesaulniers wrote:
> > > > I'm not sure that I need to handle this case (of a callbr with the same indirect destination as the default destination).  That might result from optimizations, but I don't think clang can or will generate such IR.  Maybe I should turn this into an: `assert(CBR->getIndirectDest(i) != CBR->getDefaultDest(i) && "We already emitted asm stores in the default dest");`?
> > > Yeah, it's impossible. While a CallBrInst can have a same indirect destination as the default destination as a result of optimizations, above we always create an `"asm.fallthrough"` block that is distinct from any of the named labels from the `asm goto`.
> > It could only happen with something like this:
> > 
> > ```
> > asm goto (... :::: indirect);
> > 
> > indirect:;
> > ```
> > 
> > In that simplified case, it looks like clang emits this:
> > 
> > ```
> > define dso_local void @foo() #0 {
> >   callbr void asm sideeffect "", "i,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %2)) #1
> >           to label %1 [label %2], !srcloc !6
> > 
> > 1:                                                ; preds = %0
> >   br label %2
> > 
> > 2:                                                ; preds = %1, %0
> >   ret void
> > }
> > ```
> > 
> > So it's probably safe to make this an assert.
> Oh, it's not just the default and indirects sharing, but what if the same indirect is listed twice?  I should add a test for that...i.e.
> 
> ```
> asm goto (... :::: indirect,indirect);
> ```
Oh, right, we don't allow that.

```
error: duplicate use of asm operand name "indirect"
note: asm operand name "indirect" first referenced here
```

I'm going to leave these asserts out and mark this thread done, but please reopen this thread with a new comment if you feel strongly that I should add it back (`assert(CBR->getIndirectDest(i) != CBR->getDefaultDest(i) && "We already emitted asm stores in the default dest");`).


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https://reviews.llvm.org/D136497