[CodeGen][X86] LiveRangeShrink: fix increment after end

[MuellerMP]

This fixes the infinite loop discovered in #114195.
Since we skip debug instructions at the start of the loop we do not need to skip them again at the end of the loop.

[llvmbot]

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-llvm-regalloc

Author: Mirko (MuellerMP)

Changes

This fixes the infinite loop discovered in #114195.
Since we skip debug instructions at the start of the loop we do not need to skip them again at the end of the loop.

---

Full diff: https://github.com/llvm/llvm-project/pull/115276.diff

1 Files Affected:

• (modified) llvm/lib/CodeGen/LiveRangeShrink.cpp (+1-1)

diff --git a/llvm/lib/CodeGen/LiveRangeShrink.cpp b/llvm/lib/CodeGen/LiveRangeShrink.cpp
index f4bf47c3ae158f..6a0b918d5e6f67 100644
--- a/llvm/lib/CodeGen/LiveRangeShrink.cpp
+++ b/llvm/lib/CodeGen/LiveRangeShrink.cpp
@@ -246,7 +246,7 @@ bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
         if (MI.getOperand(0).isReg())
           for (; EndIter != MBB.end() && EndIter->isDebugValue() &&
                  EndIter->hasDebugOperandForReg(MI.getOperand(0).getReg());
-               ++EndIter, ++Next)
+               ++EndIter)
             IOM[&*EndIter] = NewOrder;
         MBB.splice(I, &MBB, MI.getIterator(), EndIter);
       }

[arsenm]

Needs test

[nathanchance]

Thanks, this appears to resolve the issue I reported, but I understand there needs to be a test added before it can be merged.

[MuellerMP]

This is how far i could reduce the orginal sample. (Still repros on trunk https://llvm.godbolt.org/z/bh8boEvff)
It requires the debug value + phi setup to trigger this specific issue with debug values at the end of an MBB.