[DAGCombiner] Fix invalid alias analysis.

[courbet]

Summary

When we know for sure whether two addresses do or do not alias, we
should immediately return from DAGCombiner::isAlias().

I think this comes from a bad copy/paste, Sorry for not catching that during the
code review.

Fixes PR41855.

[courbet]

Rebase.

[courbet]

llvm/test/CodeGen/X86/lifetime-alias.ll

44

Note how the string size for "FunnyItWorkedLastTime" was incorrectly set to 15 before, and is now correctly set to 21.

[courbet]

Thanks.

[echristo]

llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

19802 ↗

(On Diff #199223)

Can you comment this? Given it's already caused a problem and isn't obvious what's going on necessarily.

[courbet]

llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

19802 ↗

(On Diff #199223)

Sure.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

static unsigned LittleEndianByteAt(unsigned BW, unsigned i) {
   return i;
 }
 
 static unsigned BigEndianByteAt(unsigned BW, unsigned i) {
   return BW - i - 1;
 }
 
 // Check if the bytes offsets we are looking at match with either big or
-// little endian value loaded. Return true for big endian, false for little 
+// little endian value loaded. Return true for big endian, false for little
 // endian, and None if match failed.
 static Optional<bool> isBigEndian(const SmallVector<int64_t, 4> &ByteOffsets,
                                   int64_t FirstOffset) {
   // The endian can be decided only when it is 2 bytes at least.
   unsigned Width = ByteOffsets.size();
   if (Width < 2)
     return None;
 
bool DAGCombiner::isAlias(SDNode *Op0, SDNode *Op1) const {
   if (MUC0.MMO && MUC1.MMO) {
     if ((MUC0.MMO->isInvariant() && MUC1.MMO->isStore()) ||
         (MUC1.MMO->isInvariant() && MUC0.MMO->isStore()))
       return false;
   }
 
   bool IsAlias;
   if (BaseIndexOffset::computeAliasing(Op0, MUC0.NumBytes, Op1, MUC1.NumBytes,
-                                       DAG, IsAlias) &&
-      !IsAlias)
+                                       DAG, IsAlias))
     return IsAlias;
 
   // The following all rely on MMO0 and MMO1 being valid. Fail conservatively if
   // either are not known.
   if (!MUC0.MMO || !MUC1.MMO)
     return true;
 
   // If one operation reads from invariant memory, and the other may store, they

llvm/test/CodeGen/X86/lifetime-alias.ll

 ; CHECK-NEXT:    movw $5632, {{[0-9]+}}(%rsp) # imm = 0x1600
 ; CHECK-NEXT:    xorps %xmm0, %xmm0
 ; CHECK-NEXT:    movaps %xmm0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movabsq $8389209137051166804, %rax # imm = 0x746C754320656854
 ; CHECK-NEXT:    movq %rax, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movl $1701999988, -{{[0-9]+}}(%rsp) # imm = 0x65727574
 ; CHECK-NEXT:    movb $0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movaps %xmm0, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    movabsq $1513209474796486656, %rax # imm = 0x1500000000000000
+; CHECK-NEXT:    movq $0, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movb $21, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movabsq $7308613581744070988, %rax # imm = 0x656D69547473614C
 ; CHECK-NEXT:    movq %rax, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movups {{.*}}(%rip), %xmm1
 ; CHECK-NEXT:    movaps %xmm1, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    movabsq $7308613581744070988, %rcx # imm = 0x656D69547473614C
-; CHECK-NEXT:    movq %rcx, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    movabsq $792633534417207296, %rcx # imm = 0xB00000000000000
-; CHECK-NEXT:    movq %rcx, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    movq -{{[0-9]+}}(%rsp), %rcx
-; CHECK-NEXT:    movq %rcx, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    movzwl -{{[0-9]+}}(%rsp), %ecx
-; CHECK-NEXT:    movw %cx, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    movb -{{[0-9]+}}(%rsp), %cl
-; CHECK-NEXT:    movb %cl, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    movb -{{[0-9]+}}(%rsp), %cl
-; CHECK-NEXT:    movb %cl, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    movl -{{[0-9]+}}(%rsp), %ecx
-; CHECK-NEXT:    movl %ecx, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movb $0, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movabsq $792633534417207296, %rax # imm = 0xB00000000000000
+; CHECK-NEXT:    movq %rax, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movq -{{[0-9]+}}(%rsp), %rax
+; CHECK-NEXT:    movq %rax, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movzwl -{{[0-9]+}}(%rsp), %eax
+; CHECK-NEXT:    movw %ax, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movb -{{[0-9]+}}(%rsp), %al
+; CHECK-NEXT:    movb %al, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movb -{{[0-9]+}}(%rsp), %al
+; CHECK-NEXT:    movb %al, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movl -{{[0-9]+}}(%rsp), %eax
+; CHECK-NEXT:    movl %eax, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movaps %xmm0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movq $0, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movq -{{[0-9]+}}(%rsp), %rax
 ; CHECK-NEXT:    movq %rax, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movaps -{{[0-9]+}}(%rsp), %xmm1
 ; CHECK-NEXT:    movups %xmm1, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movaps %xmm0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movq $0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    leaq -{{[0-9]+}}(%rsp), %rax
 ; CHECK-NEXT:    movq %rax, {{.*}}(%rip)
 ; CHECK-NEXT:    leaq -{{[0-9]+}}(%rsp), %rax