[PATCH] D139565: [IR] add new callbrpad instruction

Wed Dec 7 15:20:25 PST 2022

nickdesaulniers added a comment.

In D139565#3979659 <https://reviews.llvm.org/D139565#3979659>, @nickdesaulniers wrote:

> we'd need either ... or multiple defs of the same virt reg, which I _think_ MIR might actually support.

I tried this quickly (via `llc -stop-after=finalize-isel; <hand modify mir>; llc -start-after=finalize-isel;`. Looks like this triggers an assertion immediately:
-mtriple=aarch64:

> AArch64StackTaggingPreRA::runOnMachineFunction(llvm::MachineFunction &): Assertion `MRI->isSSA()' failed.

-mtriple=x86_64:

> MachineFunctionProperties required by Machine Common Subexpression Elimination pass are not met by function test5.

So it's not legal for selectiondag to produce multiple definitions of the same virt reg on different paths. (I guess I'm not really sure why `MachineInstr::defs()` returns an iterator then, implying there's possibly more than one def of a virtreg).

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https://reviews.llvm.org/D139565