[PATCH] D97568: [ARM] support symbolic expressions as branch target in b.w

[Nick Desaulniers via Phabricator via llvm-commits]

nickdesaulniers added inline comments.


================
Comment at: llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp:7967
     int Op = (Operands[2]->isImm()) ? 2 : 3;
     if (!static_cast<ARMOperand &>(*Operands[Op]).isSignedOffset<20, 1>())
       return Error(Operands[Op]->getStartLoc(), "branch target out of range");
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nickdesaulniers wrote:
> It's definitely not great seeing this problem across multiple different instructions; in particular I suspect that `t2Bcc` is the same operation as `t2B` (branch), just with a condition code (cc) and smaller immediate.
> 
> I still think the callers should defer checks for `MCBinaryExpr` (as this patch does), but then seeing things like `isThumbBranchTarget`/`isARMBranchTarget` makes me nervous that we can't validate `MCBinaryExpr` operands until after adjusting fixups (very late) for many other instructions.
> 
> If `MCSymbolRefExpr` is just the `.` operator in assembler, and `MCBinaryExpr` is for expressions, I wonder if `isUnsignedOffset`, `isSignedOffset`, `isLEOffset`, and `isThumbMemPC` should all return `true` for `MCBinaryExpr`?
> 
> `isARMBranchTarget`/`isThumbBranchTarget` default to `true` (for example for `MCSymbolRefExpr` and `MCBinaryExpr`, but the list above do not.  And that's curious to me.  We can't know if these immediates satisfy some range constraints until after fixup; right now these methods are a mix of what they permit when expressions involving `.` are used.
And part of that is that these `isSignedOffset` and friends all return `bool`, when for `MCSymbolRefExpr` and `MCBinaryExpr` operands the answer has a third possible state which is "maybe, but I won't know until adjusting fixups" which is one of the last things the assembler does, without rerunning instruction validation.  isUnsignedOffset, isSignedOffset, isLEOffset, and isThumbMemPC all respond "no" conservatively; isARMBranchTarget/isThumbBranchTarget respond "sure" perhaps too eagerly.  I suspect given a `MCSymbolRefExpr` or `MCBinaryExpr`, isARMBranchTarget/isThumbBranchTarget both respond true, when after fixups the answer perhaps should have been "no."


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  https://reviews.llvm.org/D97568/new/

https://reviews.llvm.org/D97568