[ConstantFold][GlobalOpt] Don't create x86_mmx null value

This fixes the assertion failure reported at
https://reviews.llvm.org/D114889#3198921 with a straightforward
check, until the cleaner fix in D115924 can be reapplied.

Author: nikic

llvm/lib/Analysis/ConstantFolding.cpp

@@ -708,7 +708,8 @@ Constant *llvm::ConstantFoldLoadFromConstPtr(Constant *C, Type *Ty,
   // is all undef or zero, we know what it loads.
   if (auto *GV = dyn_cast<GlobalVariable>(getUnderlyingObject(C))) {
     if (GV->isConstant() && GV->hasDefinitiveInitializer()) {
-      if (GV->getInitializer()->isNullValue())
+      if (GV->getInitializer()->isNullValue() && !Ty->isX86_MMXTy() &&
+          !Ty->isX86_AMXTy())
         return Constant::getNullValue(Ty);
       if (isa<UndefValue>(GV->getInitializer()))
         return UndefValue::get(Ty);

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -305,8 +305,9 @@ static bool CleanupConstantGlobalUsers(GlobalVariable *GV,
     else if (auto *LI = dyn_cast<LoadInst>(U)) {
       // A load from zeroinitializer is always zeroinitializer, regardless of
       // any applied offset.
-      if (Init->isNullValue()) {
-        LI->replaceAllUsesWith(Constant::getNullValue(LI->getType()));
+      Type *Ty = LI->getType();
+      if (Init->isNullValue() && !Ty->isX86_MMXTy() && !Ty->isX86_AMXTy()) {
+        LI->replaceAllUsesWith(Constant::getNullValue(Ty));
         EraseFromParent(LI);
         continue;
       }
@@ -316,8 +317,7 @@ static bool CleanupConstantGlobalUsers(GlobalVariable *GV,
       PtrOp = PtrOp->stripAndAccumulateConstantOffsets(
           DL, Offset, /* AllowNonInbounds */ true);
       if (PtrOp == GV) {
-        if (auto *Value = ConstantFoldLoadFromConst(Init, LI->getType(),
-                                                    Offset, DL)) {
+        if (auto *Value = ConstantFoldLoadFromConst(Init, Ty, Offset, DL)) {
           LI->replaceAllUsesWith(Value);
           EraseFromParent(LI);
         }

llvm/test/Transforms/GlobalOpt/x86_mmx_load.ll

@@ -0,0 +1,12 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -globalopt < %s | FileCheck %s
+
+@m64 = internal global <1 x i64> zeroinitializer
+
+define i32 @load_mmx() {
+; CHECK-LABEL: @load_mmx(
+; CHECK-NEXT:    ret i32 0
+;
+  %temp = load x86_mmx, x86_mmx* bitcast (<1 x i64>* @m64 to x86_mmx*)
+  ret i32 0
+}

llvm/test/Transforms/InstSimplify/ConstProp/loads.ll

@@ -280,3 +280,16 @@ define { i64, i64 } @test_load_struct() {
   %v = load { i64, i64 }, { i64, i64 }* @g3
   ret { i64, i64 } %v
 }
+
+@m64 = internal constant [2 x i64] zeroinitializer
+@idx = external global i32
+
+; This should not try to create an x86_mmx null value.
+define x86_mmx @load_mmx() {
+; CHECK-LABEL: @load_mmx(
+; CHECK-NEXT:    [[TEMP:%.*]] = load x86_mmx, x86_mmx* bitcast (i64* getelementptr ([2 x i64], [2 x i64]* @m64, i64 0, i64 ptrtoint (i32* @idx to i64)) to x86_mmx*), align 8
+; CHECK-NEXT:    ret x86_mmx [[TEMP]]
+;
+  %temp = load x86_mmx, x86_mmx* bitcast (i64* getelementptr ([2 x i64], [2 x i64]* @m64, i64 0, i64 ptrtoint (i32* @idx to i64)) to x86_mmx*)
+  ret x86_mmx %temp
+}