[llvm] [ARM][ConstantIslands] Correct MinNoSplitDisp calculation (PR #114590)

[Simon Tatham via llvm-commits]

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@@ -1324,7 +1324,8 @@ bool ARMConstantIslands::findAvailableWater(CPUser &U, unsigned UserOffset,
   MachineBasicBlock *UserBB = U.MI->getParent();
   BBInfoVector &BBInfo = BBUtils->getBBInfo();
   const Align CPEAlign = getCPEAlign(U.CPEMI);
-  unsigned MinNoSplitDisp = BBInfo[UserBB->getNumber()].postOffset(CPEAlign);
+  unsigned MinNoSplitDisp =
+      BBInfo[UserBB->getNumber()].postOffset(CPEAlign) - UserOffset;
   if (CloserWater && MinNoSplitDisp > U.getMaxDisp() / 2)
     return false;
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statham-arm wrote:

I understand that this change means that `MinNoSplitDisp` is now the exact offset between the user of the constant-pool entry and the place we're considering putting the entry itself. But why are we then splitting the block if that's more than _half_ of `U.getMaxDisp()`? I would expect that if the distance we have to reach is actually _larger_ than `U.getMaxDisp()`, that's when we need to split something.

But I don't understand why the previous code was comparing against `U.getMaxDisp() / 2` either. Do you?

https://github.com/llvm/llvm-project/pull/114590