[MIPS]: Rework atomic max/min expand for subword

The current code is so buggy: it can work for few cases.
The problems include:
  1. ll/sc works on a whole word, while other parts other than we rmw
     are dropped.
  2. The oprands are not well zero-extended for unsigned ops.
  3. It doesn't work for big-endian, as the postion of subword differs
     with little endian.

Author: wzssyqa

llvm/lib/Target/Mips/MipsExpandPseudo.cpp

@@ -342,6 +342,7 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
   bool IsMin = false;
   bool IsMax = false;
   bool IsUnsigned = false;
+  bool DestOK = false;
 
   unsigned Opcode = 0;
   switch (I->getOpcode()) {
@@ -388,6 +389,7 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
     Opcode = Mips::XOR;
     break;
   case Mips::ATOMIC_LOAD_UMIN_I8_POSTRA:
+    SEOp = Mips::SEB;
     IsUnsigned = true;
     IsMin = true;
     break;
@@ -403,6 +405,7 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
     IsMin = true;
     break;
   case Mips::ATOMIC_LOAD_UMAX_I8_POSTRA:
+    SEOp = Mips::SEB;
     IsUnsigned = true;
     IsMax = true;
     break;
@@ -473,48 +476,36 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
     unsigned SELOldVal = IsMax ? SELEQZ : SELNEZ;
     unsigned MOVIncr = IsMax ? MOVN : MOVZ;
 
-    // For little endian we need to clear uninterested bits.
-    if (STI->isLittle()) {
-      if (!IsUnsigned) {
-        BuildMI(loopMBB, DL, TII->get(Mips::SRAV), OldVal)
-            .addReg(OldVal)
-            .addReg(ShiftAmnt);
-        BuildMI(loopMBB, DL, TII->get(Mips::SRAV), Incr)
-            .addReg(Incr)
-            .addReg(ShiftAmnt);
-        if (STI->hasMips32r2()) {
-          BuildMI(loopMBB, DL, TII->get(SEOp), OldVal).addReg(OldVal);
-          BuildMI(loopMBB, DL, TII->get(SEOp), Incr).addReg(Incr);
-        } else {
-          const unsigned ShiftImm = SEOp == Mips::SEH ? 16 : 24;
-          BuildMI(loopMBB, DL, TII->get(Mips::SLL), OldVal)
-              .addReg(OldVal, RegState::Kill)
-              .addImm(ShiftImm);
-          BuildMI(loopMBB, DL, TII->get(Mips::SRA), OldVal)
-              .addReg(OldVal, RegState::Kill)
-              .addImm(ShiftImm);
-          BuildMI(loopMBB, DL, TII->get(Mips::SLL), Incr)
-              .addReg(Incr, RegState::Kill)
-              .addImm(ShiftImm);
-          BuildMI(loopMBB, DL, TII->get(Mips::SRA), Incr)
-              .addReg(Incr, RegState::Kill)
-              .addImm(ShiftImm);
-        }
-      } else {
-        // and OldVal, OldVal, Mask
-        // and Incr, Incr, Mask
-        BuildMI(loopMBB, DL, TII->get(Mips::AND), OldVal)
-            .addReg(OldVal)
-            .addReg(Mask);
-        BuildMI(loopMBB, DL, TII->get(Mips::AND), Incr)
-            .addReg(Incr)
-            .addReg(Mask);
-      }
+    BuildMI(loopMBB, DL, TII->get(Mips::SRAV), StoreVal)
+        .addReg(OldVal)
+        .addReg(ShiftAmnt);
+    if (STI->hasMips32r2() && !IsUnsigned) {
+      BuildMI(loopMBB, DL, TII->get(SEOp), StoreVal).addReg(StoreVal);
+    } else if (STI->hasMips32r2() && IsUnsigned) {
+      const unsigned OpMask = SEOp == Mips::SEH ? 0xffff : 0xff;
+      BuildMI(loopMBB, DL, TII->get(Mips::ANDi), StoreVal).addReg(StoreVal).addImm(OpMask);
+    } else {
+      const unsigned ShiftImm = SEOp == Mips::SEH ? 16 : 24;
+      const unsigned SROp = IsUnsigned ? Mips::SRL : Mips::SRA;
+      BuildMI(loopMBB, DL, TII->get(Mips::SLL), StoreVal)
+          .addReg(StoreVal, RegState::Kill)
+          .addImm(ShiftImm);
+      BuildMI(loopMBB, DL, TII->get(SROp), StoreVal)
+          .addReg(StoreVal, RegState::Kill)
+          .addImm(ShiftImm);
     }
-    // unsigned: sltu Scratch4, oldVal, Incr
-    // signed:   slt Scratch4, oldVal, Incr
+    BuildMI(loopMBB, DL, TII->get(Mips::OR), Dest)
+        .addReg(Mips::ZERO)
+        .addReg(StoreVal);
+    DestOK = true;
+    BuildMI(loopMBB, DL, TII->get(Mips::SLLV), StoreVal)
+        .addReg(StoreVal)
+        .addReg(ShiftAmnt);
+
+    // unsigned: sltu Scratch4, StoreVal, Incr
+    // signed:   slt Scratch4, StoreVal, Incr
     BuildMI(loopMBB, DL, TII->get(SLTScratch4), Scratch4)
-        .addReg(OldVal)
+        .addReg(StoreVal)
         .addReg(Incr);
 
     if (STI->hasMips64r6() || STI->hasMips32r6()) {
@@ -525,7 +516,7 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
       //      seleqz Scratch4, Incr, Scratch4
       //      or BinOpRes, BinOpRes, Scratch4
       BuildMI(loopMBB, DL, TII->get(SELOldVal), BinOpRes)
-          .addReg(OldVal)
+          .addReg(StoreVal)
           .addReg(Scratch4);
       BuildMI(loopMBB, DL, TII->get(SELIncr), Scratch4)
           .addReg(Incr)
@@ -534,12 +525,12 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
           .addReg(BinOpRes)
           .addReg(Scratch4);
     } else {
-      // max: move BinOpRes, OldVal
+      // max: move BinOpRes, StoreVal
       //      movn BinOpRes, Incr, Scratch4, BinOpRes
-      // min: move BinOpRes, OldVal
+      // min: move BinOpRes, StoreVal
       //      movz BinOpRes, Incr, Scratch4, BinOpRes
       BuildMI(loopMBB, DL, TII->get(OR), BinOpRes)
-          .addReg(OldVal)
+          .addReg(StoreVal)
           .addReg(Mips::ZERO);
       BuildMI(loopMBB, DL, TII->get(MOVIncr), BinOpRes)
           .addReg(Incr)
@@ -586,23 +577,24 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
   //    srl     srlres,maskedoldval1,shiftamt
   //    sign_extend dest,srlres
 
-  sinkMBB->addSuccessor(exitMBB, BranchProbability::getOne());
-
-  BuildMI(sinkMBB, DL, TII->get(Mips::AND), Dest)
-    .addReg(OldVal).addReg(Mask);
-  BuildMI(sinkMBB, DL, TII->get(Mips::SRLV), Dest)
-      .addReg(Dest).addReg(ShiftAmnt);
+  if (!DestOK) {
+    sinkMBB->addSuccessor(exitMBB, BranchProbability::getOne());
+    BuildMI(sinkMBB, DL, TII->get(Mips::AND), Dest).addReg(OldVal).addReg(Mask);
+    BuildMI(sinkMBB, DL, TII->get(Mips::SRLV), Dest)
+        .addReg(Dest)
+        .addReg(ShiftAmnt);
 
-  if (STI->hasMips32r2()) {
-    BuildMI(sinkMBB, DL, TII->get(SEOp), Dest).addReg(Dest);
-  } else {
-    const unsigned ShiftImm = SEOp == Mips::SEH ? 16 : 24;
-    BuildMI(sinkMBB, DL, TII->get(Mips::SLL), Dest)
-        .addReg(Dest, RegState::Kill)
-        .addImm(ShiftImm);
-    BuildMI(sinkMBB, DL, TII->get(Mips::SRA), Dest)
-        .addReg(Dest, RegState::Kill)
-        .addImm(ShiftImm);
+    if (STI->hasMips32r2()) {
+      BuildMI(sinkMBB, DL, TII->get(SEOp), Dest).addReg(Dest);
+    } else {
+      const unsigned ShiftImm = SEOp == Mips::SEH ? 16 : 24;
+      BuildMI(sinkMBB, DL, TII->get(Mips::SLL), Dest)
+          .addReg(Dest, RegState::Kill)
+          .addImm(ShiftImm);
+      BuildMI(sinkMBB, DL, TII->get(Mips::SRA), Dest)
+          .addReg(Dest, RegState::Kill)
+          .addImm(ShiftImm);
+    }
   }
 
   LivePhysRegs LiveRegs;