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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-arm

Author: None (pzhengqc)

<details>
<summary>Changes</summary>

MinNoSplitDisp was first introduced in D16890 to handle cases where the ConstantIslands pass fails to converge in the presence of big basic blocks. However, the computation of the variable seems to be wrong as it currently computes the offset immediately following UserBB. In other words, it represents the distance from the beginning of the function to the end of UserBB. The distance from the beginning of the function does not seem to be a good indicator of how big the basic block is unless the basic block is close to the beginning of the function. I think MinNoSplitDisp should compute the distance between UserOffset and the end of UserBB instead.

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


1 Files Affected:

- (modified) llvm/lib/Target/ARM/ARMConstantIslandPass.cpp (+2-1) 


``````````diff
diff --git a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
index 1312b44b49bdcc..e0575f7c125f67 100644
--- a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -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;
   for (water_iterator IP = std::prev(WaterList.end()), B = WaterList.begin();;

``````````

</details>


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