[llvm] [RISCV] Remove incomplete PRE_DEC/POST_DEC code for XTHeadMemIdx. (PR #76922)

[Craig Topper via llvm-commits]

https://github.com/topperc created https://github.com/llvm/llvm-project/pull/76922

As far as I can tell if getIndexedAddressParts received an ISD::SUB, the constant would be negated. So `IsInc` should be set to true since the SUB was effectively converted to ADD.  This means we should never use PRE_DEC/POST_DEC.

No tests are affected because DAGCombine aggressively turns SUB with constant into ADD so no lit test has a SUB reach getIndexedAddressParts.

>From 90a29ac6da6f637515d2d28a933ef2645159f42e Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Thu, 4 Jan 2024 00:43:31 -0800
Subject: [PATCH] [RISCV] Remove incomplete PRE_DEC/POST_DEC code for
 XTHeadMemIdx.

As far as I can tell if getIndexedAddressParts received an ISD::SUB,
the constant would be negated. So IsInc should be set to true since
the SUB was effectively converted to ADD.  This means we should never
use PRE_DEC/POST_DEC.

No tests are affected because DAGCombine aggressively turns SUB with
constant into ADD so no lit test has a SUB reach getIndexedAddressParts.
---
 llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp | 10 ++++------
 llvm/lib/Target/RISCV/RISCVISelLowering.cpp |  6 +++---
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
index bfa3bf3cc74e2b..befa9e1159bef1 100644
--- a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
@@ -763,14 +763,12 @@ bool RISCVDAGToDAGISel::tryIndexedLoad(SDNode *Node) {
     return false;
 
   EVT LoadVT = Ld->getMemoryVT();
-  bool IsPre = (AM == ISD::PRE_INC || AM == ISD::PRE_DEC);
-  bool IsPost = (AM == ISD::POST_INC || AM == ISD::POST_DEC);
+  assert(AM == ISD::PRE_INC || AM == ISD::POST_INC &&
+         "Unexpected addressing mode");
+  bool IsPre = AM == ISD::PRE_INC;
+  bool IsPost = AM == ISD::POST_INC;
   int64_t Offset = C->getSExtValue();
 
-  // Convert decrements to increments by a negative quantity.
-  if (AM == ISD::PRE_DEC || AM == ISD::POST_DEC)
-    Offset = -Offset;
-
   // The constants that can be encoded in the THeadMemIdx instructions
   // are of the form (sign_extend(imm5) << imm2).
   int64_t Shift;
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index c8a94adcd91c6a..0a886fe70eeaed 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -1350,8 +1350,7 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
   }
 
   if (Subtarget.hasVendorXTHeadMemIdx()) {
-    for (unsigned im = (unsigned)ISD::PRE_INC; im != (unsigned)ISD::POST_DEC;
-         ++im) {
+    for (unsigned im : {ISD::PRE_INC, ISD::POST_INC}) {
       setIndexedLoadAction(im, MVT::i8, Legal);
       setIndexedStoreAction(im, MVT::i8, Legal);
       setIndexedLoadAction(im, MVT::i16, Legal);
@@ -19288,7 +19287,8 @@ bool RISCVTargetLowering::getIndexedAddressParts(SDNode *Op, SDValue &Base,
     if (!isLegalIndexedOffset)
       return false;
 
-    IsInc = (Op->getOpcode() == ISD::ADD);
+    // Constant for SUB was negated earlier.
+    IsInc = true;
     Offset = Op->getOperand(1);
     return true;
   }