[llvm] [DAG] Add ComputeNumSignBits(FREEZE(X)) handling (PR #161507)

[Björn Pettersson via llvm-commits]

https://github.com/bjope updated https://github.com/llvm/llvm-project/pull/161507

>From 6f29ad5974e2f1190fa05e901b50a6c6433a72d8 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev at redking.me.uk>
Date: Wed, 1 Oct 2025 12:11:57 +0100
Subject: [PATCH] [DAG] Add ComputeNumSignBits(FREEZE(X)) handling

If X is known never under/poison then skip the freeze and return ComputeNumSignBits(X)
---
 llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 5 +++++
 llvm/test/CodeGen/AArch64/freeze.ll            | 6 ------
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 8fc7eabf90ea8..95f53fe0bfdba 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -4762,6 +4762,11 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
   case ISD::AssertZext:
     Tmp = cast<VTSDNode>(Op.getOperand(1))->getVT().getSizeInBits();
     return VTBits-Tmp;
+  case ISD::FREEZE:
+    if (isGuaranteedNotToBeUndefOrPoison(Op.getOperand(0), DemandedElts,
+                                         /*PoisonOnly=*/false))
+      return ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1);
+    break;
   case ISD::MERGE_VALUES:
     return ComputeNumSignBits(Op.getOperand(Op.getResNo()), DemandedElts,
                               Depth + 1);
diff --git a/llvm/test/CodeGen/AArch64/freeze.ll b/llvm/test/CodeGen/AArch64/freeze.ll
index fae3bbe2dcfba..fb909fec90434 100644
--- a/llvm/test/CodeGen/AArch64/freeze.ll
+++ b/llvm/test/CodeGen/AArch64/freeze.ll
@@ -466,15 +466,12 @@ define <8 x i16> @freeze_urhadd(<8 x i16> %a0, <8 x i16> %a1) {
   ret <8 x i16> %masked
 }
 
-; TODO: Unnecessary sext_inreg
 define <8 x i16> @freeze_shadd(<8 x i8> %a0, <8 x i16> %a1) {
 ; CHECK-LABEL: freeze_shadd:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    sshll v0.8h, v0.8b, #0
 ; CHECK-NEXT:    sshr v1.8h, v1.8h, #8
 ; CHECK-NEXT:    shadd v0.8h, v0.8h, v1.8h
-; CHECK-NEXT:    shl v0.8h, v0.8h, #8
-; CHECK-NEXT:    sshr v0.8h, v0.8h, #8
 ; CHECK-NEXT:    ret
   %x0 = sext <8 x i8> %a0 to <8 x i16>
   %x1 = ashr <8 x i16> %a1, splat (i16 8)
@@ -485,15 +482,12 @@ define <8 x i16> @freeze_shadd(<8 x i8> %a0, <8 x i16> %a1) {
   ret <8 x i16> %sext
 }
 
-; TODO: Unnecessary sext_inreg
 define <8 x i16> @freeze_srhadd(<8 x i8> %a0, <8 x i16> %a1) {
 ; CHECK-LABEL: freeze_srhadd:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    sshll v0.8h, v0.8b, #0
 ; CHECK-NEXT:    sshr v1.8h, v1.8h, #8
 ; CHECK-NEXT:    srhadd v0.8h, v0.8h, v1.8h
-; CHECK-NEXT:    shl v0.8h, v0.8h, #8
-; CHECK-NEXT:    sshr v0.8h, v0.8h, #8
 ; CHECK-NEXT:    ret
   %x0 = sext <8 x i8> %a0 to <8 x i16>
   %x1 = ashr <8 x i16> %a1, splat (i16 8)