[llvm-branch-commits] [llvm] [AArch64] Add new pass after VirtRegRewriter to add implicit-defs (PR #174188)

[Gaëtan Bossu via llvm-branch-commits]

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+//===- AArch64SRLTDefineSuperRegs.cpp -------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// When SubRegister Liveness Tracking (SRLT) is enabled, this pass adds
+// extra implicit-def's to instructions that define the low N bits of
+// a GPR/FPR register to also define the top bits, because all AArch64
+// instructions that write the low bits of a GPR/FPR also implicitly zero
+// the top bits.  For example, 'mov w0, w1' writes zeroes to the top 32-bits of
+// x0, so this pass adds a `implicit-def $x0` after register allocation.
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gbossu wrote:

Would it hurt to run the pass even when SRLT is disabled?

https://github.com/llvm/llvm-project/pull/174188