[Mips] Fix missing sign extension in expansion of sub-word atomic max

Add sign extension "SEB/SEH" before compare.

Fix #61881

Author: yingopq

llvm/lib/Target/Mips/MipsExpandPseudo.cpp

@@ -388,18 +388,32 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
     Opcode = Mips::XOR;
     break;
   case Mips::ATOMIC_LOAD_UMIN_I8_POSTRA:
+    IsUnsigned = true;
+    IsMin = true;
+    break;
   case Mips::ATOMIC_LOAD_UMIN_I16_POSTRA:
     IsUnsigned = true;
-    [[fallthrough]];
+    IsMin = true;
+    break;
   case Mips::ATOMIC_LOAD_MIN_I8_POSTRA:
+    SEOp = Mips::SEB;
+    IsMin = true;
+    break;
   case Mips::ATOMIC_LOAD_MIN_I16_POSTRA:
     IsMin = true;
     break;
   case Mips::ATOMIC_LOAD_UMAX_I8_POSTRA:
+    IsUnsigned = true;
+    IsMax = true;
+    break;
   case Mips::ATOMIC_LOAD_UMAX_I16_POSTRA:
     IsUnsigned = true;
-    [[fallthrough]];
+    IsMax = true;
+    break;
   case Mips::ATOMIC_LOAD_MAX_I8_POSTRA:
+    SEOp = Mips::SEB;
+    IsMax = true;
+    break;
   case Mips::ATOMIC_LOAD_MAX_I16_POSTRA:
     IsMax = true;
     break;
@@ -467,8 +481,34 @@ bool MipsExpandPseudo::expandAtomicBinOpSubword(
           .addReg(OldVal)
           .addReg(Mask);
       BuildMI(loopMBB, DL, TII->get(Mips::AND), Incr).addReg(Incr).addReg(Mask);
-    }
 
+      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);
+        }
+      }
+    }
     // unsigned: sltu Scratch4, oldVal, Incr
     // signed:   slt Scratch4, oldVal, Incr
     BuildMI(loopMBB, DL, TII->get(SLTScratch4), Scratch4)