[llvm] [X86] Don't allocate unused padding in front of realigned locals (PR #227495)

[Evgenii Kudriashov via llvm-commits]

================
@@ -1901,9 +1927,14 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF,
     NumBytes =
         FrameSize - (X86FI->getCalleeSavedFrameSize() + TailCallArgReserveSize);
 
-    // Callee-saved registers are pushed on stack before the stack is realigned.
-    if (TRI->hasStackRealignment(MF) && !IsWin64Prologue)
-      NumBytes = alignTo(NumBytes, MaxAlign);
+    // Callee-saved registers are pushed on stack before the stack is realigned,
+    // and the realignment itself already provides the local objects' alignment,
+    // so leave out the padding PEI put in front of them for it.
+    if (TRI->hasStackRealignment(MF) && !IsWin64Prologue) {
+      uint64_t Padding = getUnusedLocalAreaPadding(MFI);
+      assert(Padding <= NumBytes && "Padding exceeds the local area");
+      NumBytes = alignTo(NumBytes - Padding, MaxAlign);
----------------
e-kud wrote:

In my mind under-allocation is more problematic than over-allocation. But I don't have any example from the top of my hand. But is still doesn't address the hackish (in my mind) way of fixing it.

https://github.com/llvm/llvm-project/pull/227495