[BOLT][RISCV] Handle long tail calls #67098
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Long tail calls use the following instruction sequence on RISC-V: ``` 1: auipc xi, %pcrel_hi(sym) jalr zero, %pcrel_lo(1b)(xi) ``` Since the second instruction in isolation looks like an indirect branch, this confused BOLT and most functions containing a long tail call got marked with "unknown control flow" and didn't get optimized as a consequence. This patch fixes this by detecting long tail call sequence in `analyzeIndirectBranch`. `FixRISCVCallsPass` also had to be updated to expand long tail calls to `PseudoTAIL` instead of `PseudoCALL`. Besides this, this patch also fixes a minor issue with compressed tail calls (`c.jr`) not being detected. Note that I had to change `BinaryFunction::postProcessIndirectBranches` slightly: the documentation of `MCPlusBuilder::analyzeIndirectBranch` mentions that the [`Begin`, `End`) range contains the instructions immediately preceding `Instruction`. However, in `postProcessIndirectBranches`, *all* the instructions in the BB where passed in the range. This made it difficult to find the preceding instruction so I made sure *only* the preceding instructions are passed.
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Long tail calls use the following instruction sequence on RISC-V:
Since the second instruction in isolation looks like an indirect branch, this confused BOLT and most functions containing a long tail call got marked with "unknown control flow" and didn't get optimized as a consequence.
This patch fixes this by detecting long tail call sequence in
analyzeIndirectBranch.FixRISCVCallsPassalso had to be updated to expand long tail calls toPseudoTAILinstead ofPseudoCALL.Besides this, this patch also fixes a minor issue with compressed tail calls (
c.jr) not being detected.Note that I had to change
BinaryFunction::postProcessIndirectBranchesslightly: the documentation ofMCPlusBuilder::analyzeIndirectBranchmentions that the [Begin,End) range contains the instructions immediately precedingInstruction. However, inpostProcessIndirectBranches, all the instructions in the BB where passed in the range. This made it difficult to find the preceding instruction so I made sure only the preceding instructions are passed.