[llvm] [X86] Clear regcall arg aliases in dynamic CSR masks (PR #225262)

[Aochang Liu via llvm-commits]

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@@ -2682,12 +2683,16 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
     unsigned RegMaskSize = MachineOperand::getRegMaskSize(TRI->getNumRegs());
     memcpy(RegMask, Mask, sizeof(RegMask[0]) * RegMaskSize);
 
-    // Make sure all sub registers of the argument registers are reset
-    // in the RegMask.
+    // Make sure all aliases of the argument registers are reset in the
+    // RegMask, including superregisters. Clearing only subregs_inclusive is
+    // insufficient: an i32 argument in R14D must also remove R14 from the
+    // preserved set, otherwise a live 64-bit value in R14 can be incorrectly
+    // kept across the call (see llvm/llvm-project#225057).
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HelloWorldU wrote:

Nit: Don't pass specific issue link, keep this comment only about its responsibility and function.

https://github.com/llvm/llvm-project/pull/225262