[llvm] [ARM] Only change mask if demanded bits says we can optimize (PR #165106)

[via llvm-commits]

https://github.com/AZero13 updated https://github.com/llvm/llvm-project/pull/165106

>From e9be61c1c74615177cfb55b9205e253b4a539059 Mon Sep 17 00:00:00 2001
From: AZero13 <gfunni234 at gmail.com>
Date: Sat, 25 Oct 2025 12:27:59 -0400
Subject: [PATCH] [ARM] Only change mask if demanded bits says we can optimize

---
 llvm/lib/Target/ARM/ARMISelLowering.cpp | 34 ++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 313ae3d68fb83..d8bc59177722e 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -20138,6 +20138,17 @@ void ARMTargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
   }
 }
 
+static bool isLegalLogicalImmediate(unsigned Imm,
+                                    const ARMSubtarget *Subtarget) {
+  // Handle special cases first
+  if (!Subtarget->isThumb())
+    return ARM_AM::getSOImmVal(Imm) != -1;
+  if (Subtarget->isThumb2())
+    return ARM_AM::getT2SOImmVal(Imm) != -1;
+  // Thumb1 only has 8-bit unsigned immediate.
+  return Imm <= 255;
+}
+
 bool ARMTargetLowering::targetShrinkDemandedConstant(
     SDValue Op, const APInt &DemandedBits, const APInt &DemandedElts,
     TargetLoweringOpt &TLO) const {
@@ -20180,6 +20191,14 @@ bool ARMTargetLowering::targetShrinkDemandedConstant(
   if (ExpandedMask == ~0U)
     return TLO.CombineTo(Op, Op.getOperand(0));
 
+  // Don't optimize if it is legal already.
+  if (isLegalLogicalImmediate(Mask, Subtarget))
+    return false;
+
+  // bic
+  if (isLegalLogicalImmediate(~Mask, Subtarget))
+    return false;
+
   auto IsLegalMask = [ShrunkMask, ExpandedMask](unsigned Mask) -> bool {
     return (ShrunkMask & Mask) == ShrunkMask && (~ExpandedMask & Mask) == 0;
   };
@@ -20200,14 +20219,16 @@ bool ARMTargetLowering::targetShrinkDemandedConstant(
   if (IsLegalMask(0xFFFF))
     return UseMask(0xFFFF);
 
-  // [1, 255] is Thumb1 movs+ands, legal immediate for ARM/Thumb2.
-  // FIXME: Prefer a contiguous sequence of bits for other optimizations.
-  if (ShrunkMask < 256)
+  if (isLegalLogicalImmediate(ShrunkMask, Subtarget))
     return UseMask(ShrunkMask);
 
-  // [-256, -2] is Thumb1 movs+bics, legal immediate for ARM/Thumb2.
-  // FIXME: Prefer a contiguous sequence of bits for other optimizations.
-  if ((int)ExpandedMask <= -2 && (int)ExpandedMask >= -256)
+  if (isLegalLogicalImmediate(~ShrunkMask, Subtarget))
+    return UseMask(ShrunkMask);
+
+  if (isLegalLogicalImmediate(ExpandedMask, Subtarget))
+    return UseMask(ExpandedMask);
+
+  if (isLegalLogicalImmediate(~ExpandedMask, Subtarget))
     return UseMask(ExpandedMask);
 
   // Potential improvements:
@@ -20215,7 +20236,6 @@ bool ARMTargetLowering::targetShrinkDemandedConstant(
   // We could try to recognize lsls+lsrs or lsrs+lsls pairs here.
   // We could try to prefer Thumb1 immediates which can be lowered to a
   // two-instruction sequence.
-  // We could try to recognize more legal ARM/Thumb2 immediates here.
 
   return false;
 }