[llvm] [Mips][BranchFolding] Enable safe tail merging (PR #225382)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-mips

Author: Jiaxun Yang (FlyGoat)

<details>
<summary>Changes</summary>

MIPS disables tail merging when the long-branch pass is enabled because branch expansion may clobber $at. This also blocks merges where $at is dead at the new branch.

Use physical-register liveness to reject tails that need $at, allowing safe merges for standard MIPS and microMIPS. Keep MIPS16 tail merging disabled because its far branches clobber $ra.

Check both candidate tails before selecting a common tail. The existing split check is only reached when creating a new block; reusing an entire block can still introduce a branch without passing that check.

Assisted-by: chatgpt-5.6-luna # Testcases

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Full diff: https://github.com/llvm/llvm-project/pull/225382.diff


11 Files Affected:

- (modified) llvm/lib/CodeGen/BranchFolding.cpp (+5-1) 
- (modified) llvm/lib/Target/Mips/MipsInstrInfo.cpp (+19) 
- (modified) llvm/lib/Target/Mips/MipsInstrInfo.h (+3) 
- (modified) llvm/lib/Target/Mips/MipsTargetMachine.cpp (-5) 
- (modified) llvm/test/CodeGen/Mips/compact-branch-combine-never.ll (+3-2) 
- (modified) llvm/test/CodeGen/Mips/compact-branch-combine.ll (+3-2) 
- (removed) llvm/test/CodeGen/Mips/disable-tail-merge.ll (-33) 
- (modified) llvm/test/CodeGen/Mips/llvm-ir/forbidden-slot-ir.ll (+2-1) 
- (modified) llvm/test/CodeGen/Mips/mips1-load-in-delay-slot.ll (+2-1) 
- (modified) llvm/test/CodeGen/Mips/pseudo-jump-fill.ll (+4-7) 
- (added) llvm/test/CodeGen/Mips/tail-merge.ll (+88) 


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</details>


https://github.com/llvm/llvm-project/pull/225382