[llvm] [X86][ARM] Invert the low bit to get the inverse predicate (NFC) (PR #151748)

[via llvm-commits]

https://github.com/AZero13 created https://github.com/llvm/llvm-project/pull/151748

Both ARM and x86 defined their predicate in such a way to allow bit twiddling to get inverse predicates

>From 241e041194c7eecf68428546e24da2e336923b9f Mon Sep 17 00:00:00 2001
From: Rose <gfunni234 at gmail.com>
Date: Fri, 1 Aug 2025 14:42:41 -0400
Subject: [PATCH] [X86][ARM] Invert the low bit to get the inverse predicate
 (NFC)

Both ARM and x86 defined their predicate in such a way to allow bit twiddling to get inverse predicates
---
 llvm/lib/Target/ARM/Utils/ARMBaseInfo.h | 20 ++----------
 llvm/lib/Target/X86/X86InstrInfo.cpp    | 43 ++-----------------------
 llvm/lib/Target/X86/X86InstrInfo.h      |  6 +++-
 3 files changed, 11 insertions(+), 58 deletions(-)

diff --git a/llvm/lib/Target/ARM/Utils/ARMBaseInfo.h b/llvm/lib/Target/ARM/Utils/ARMBaseInfo.h
index dc4f811e075c6..ebf4eaa122481 100644
--- a/llvm/lib/Target/ARM/Utils/ARMBaseInfo.h
+++ b/llvm/lib/Target/ARM/Utils/ARMBaseInfo.h
@@ -46,23 +46,9 @@ enum CondCodes { // Meaning (integer)          Meaning (floating-point)
 };
 
 inline static CondCodes getOppositeCondition(CondCodes CC) {
-  switch (CC) {
-  default: llvm_unreachable("Unknown condition code");
-  case EQ: return NE;
-  case NE: return EQ;
-  case HS: return LO;
-  case LO: return HS;
-  case MI: return PL;
-  case PL: return MI;
-  case VS: return VC;
-  case VC: return VS;
-  case HI: return LS;
-  case LS: return HI;
-  case GE: return LT;
-  case LT: return GE;
-  case GT: return LE;
-  case LE: return GT;
-  }
+  // To reverse a condition it's necessary to only invert the low bit:
+
+  return static_cast<CondCodes>(static_cast<unsigned>(CC) ^ 0x1);
 }
 
 /// getSwappedCondition - assume the flags are set by MI(a,b), return
diff --git a/llvm/lib/Target/X86/X86InstrInfo.cpp b/llvm/lib/Target/X86/X86InstrInfo.cpp
index abf365eedec39..833833678bb7a 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.cpp
+++ b/llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -3300,46 +3300,9 @@ unsigned X86::getNonNDVariant(unsigned Opc) {
 /// Return the inverse of the specified condition,
 /// e.g. turning COND_E to COND_NE.
 X86::CondCode X86::GetOppositeBranchCondition(X86::CondCode CC) {
-  switch (CC) {
-  default:
-    llvm_unreachable("Illegal condition code!");
-  case X86::COND_E:
-    return X86::COND_NE;
-  case X86::COND_NE:
-    return X86::COND_E;
-  case X86::COND_L:
-    return X86::COND_GE;
-  case X86::COND_LE:
-    return X86::COND_G;
-  case X86::COND_G:
-    return X86::COND_LE;
-  case X86::COND_GE:
-    return X86::COND_L;
-  case X86::COND_B:
-    return X86::COND_AE;
-  case X86::COND_BE:
-    return X86::COND_A;
-  case X86::COND_A:
-    return X86::COND_BE;
-  case X86::COND_AE:
-    return X86::COND_B;
-  case X86::COND_S:
-    return X86::COND_NS;
-  case X86::COND_NS:
-    return X86::COND_S;
-  case X86::COND_P:
-    return X86::COND_NP;
-  case X86::COND_NP:
-    return X86::COND_P;
-  case X86::COND_O:
-    return X86::COND_NO;
-  case X86::COND_NO:
-    return X86::COND_O;
-  case X86::COND_NE_OR_P:
-    return X86::COND_E_AND_NP;
-  case X86::COND_E_AND_NP:
-    return X86::COND_NE_OR_P;
-  }
+   // To reverse a condition it's necessary to only invert the low bit:
+
+  return static_cast<CondCodes>(static_cast<unsigned>(CC) ^ 0x1);
 }
 
 /// Assuming the flags are set by MI(a,b), return the condition code if we
diff --git a/llvm/lib/Target/X86/X86InstrInfo.h b/llvm/lib/Target/X86/X86InstrInfo.h
index 9dc5f4b0e086e..125e1d9efe0c4 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.h
+++ b/llvm/lib/Target/X86/X86InstrInfo.h
@@ -85,7 +85,11 @@ unsigned getNonNDVariant(unsigned Opc);
 
 /// GetOppositeBranchCondition - Return the inverse of the specified cond,
 /// e.g. turning COND_E to COND_NE.
-CondCode GetOppositeBranchCondition(CondCode CC);
+CondCode GetOppositeBranchCondition(CondCode CC) {
+   // To reverse a condition it's necessary to only invert the low bit:
+
+  return static_cast<CondCode>(static_cast<unsigned>(CC) ^ 0x1);
+}
 
 /// Get the VPCMP immediate for the given condition.
 unsigned getVPCMPImmForCond(ISD::CondCode CC);