[PATCH] D127972: [AsmPrinter] Further restrict expressions supported in global initializers

[Chris Lattner via Phabricator via llvm-commits]

lattner added a comment.

I'm not a competent reviewer for this patch, but it caused me to looked up other uses of `MCBinaryExpr::createDiv`, I see the ARM backend is doing this:

  // Otherwise it's an offset from the dispatch instruction. Construct an
  // MCExpr for the entry. We want a value of the form:
  // (BasicBlockAddr - TBBInstAddr + 4) / 2
  //
  // For example, a TBB table with entries jumping to basic blocks BB0 and BB1
  // would look like:
  // LJTI_0_0:
  //    .byte (LBB0 - (LCPI0_0 + 4)) / 2
  //    .byte (LBB1 - (LCPI0_0 + 4)) / 2
  // where LCPI0_0 is a label defined just before the TBB instruction using
  // this table.
  MCSymbol *TBInstPC = GetCPISymbol(MI->getOperand(0).getImm());
  const MCExpr *Expr = MCBinaryExpr::createAdd(
      MCSymbolRefExpr::create(TBInstPC, OutContext),
      MCConstantExpr::create(4, OutContext), OutContext);
  Expr = MCBinaryExpr::createSub(MBBSymbolExpr, Expr, OutContext);
  Expr = MCBinaryExpr::createDiv(Expr, MCConstantExpr::create(2, OutContext),
                                 OutContext);
   

This is super crazy.  MC should support target specific relocations like this.  I'm pretty sure this has to have magic pattern matching in the assembler?


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https://reviews.llvm.org/D127972