llvm-alloc-opt: handle dead code better (#39801)

In some cases (particularly after removing a phi node), we might end up
in a circumstance where it appears statically that we would use a slot
for both a ref and bits. Avoid generating malformed IR in this case.

In the future, this situation could also possibly happen if we walked
through phi nodes and attempted to merge the contents (with great care).

src/llvm-alloc-opt.cpp

@@ -350,7 +350,8 @@ void Optimizer::optimizeAll()
             if (field.hasobjref) {
                 has_ref = true;
                 // This can be relaxed a little based on hasload
-                if (field.hasaggr || field.multiloc) {
+                // TODO: add support for hasaggr load/store
+                if (field.hasaggr || field.multiloc || field.size != sizeof(void*)) {
                     has_refaggr = true;
                     break;
                 }
@@ -361,15 +362,12 @@ void Optimizer::optimizeAll()
             splitOnStack(orig);
             continue;
         }
-        if (has_ref) {
-            if (use_info.memops.size() != 1 || has_refaggr ||
-                use_info.memops.begin()->second.size != sz) {
-                if (use_info.hastypeof)
-                    optimizeTag(orig);
-                continue;
-            }
-            // The object only has a single field that's a reference with only one kind of access.
+        if (has_refaggr) {
+            if (use_info.hastypeof)
+                optimizeTag(orig);
+            continue;
         }
+        // The object has no fields with mix reference access
         moveToStack(orig, sz, has_ref);
     }
 }
@@ -444,6 +442,7 @@ Optimizer::AllocUseInfo::getField(uint32_t offset, uint32_t size, Type *elty)
         if (it->first + it->second.size >= offset + size) {
             if (it->second.elty != elty)
                 it->second.elty = nullptr;
+            assert(it->second.elty == nullptr || (it->first == offset && it->second.size == size));
             return *it;
         }
         if (it->first + it->second.size > offset) {
@@ -454,7 +453,7 @@ Optimizer::AllocUseInfo::getField(uint32_t offset, uint32_t size, Type *elty)
     else {
         it = memops.begin();
     }
-    // Now fine the last slot that overlaps with the current memory location.
+    // Now find the last slot that overlaps with the current memory location.
     // Also set `lb` if we didn't find any above.
     for (; it != end && it->first < offset + size; ++it) {
         if (lb == end)
@@ -493,8 +492,8 @@ bool Optimizer::AllocUseInfo::addMemOp(Instruction *inst, unsigned opno, uint32_
     memop.isaggr = isa<StructType>(elty) || isa<ArrayType>(elty) || isa<VectorType>(elty);
     memop.isobjref = hasObjref(elty);
     auto &field = getField(offset, size, elty);
-    if (field.first != offset || field.second.size != size)
-        field.second.multiloc = true;
+    if (field.second.hasobjref != memop.isobjref)
+        field.second.multiloc = true; // can't split this field, since it contains a mix of references and bits
     if (!isstore)
         field.second.hasload = true;
     if (memop.isobjref) {

test/llvmpasses/alloc-opt2.jl → test/llvmpasses/alloc-opt-pass.jl

@@ -107,3 +107,34 @@ declare void @llvm.memcpy.p11i8.p0i8.i64(i8 addrspace(11)* nocapture writeonly,
 declare token @llvm.julia.gc_preserve_begin(...)
 declare void @llvm.julia.gc_preserve_end(token)
 """)
+
+# CHECK-LABEL: @memref_collision
+# CHECK: call {}*** @julia.ptls_states()
+# CHECK-NOT: store {}
+# CHECK: store i
+# CHECK-NOT: store {}
+# CHECK: L1:
+# CHECK: load {}
+# CHECK: L2:
+# CHECK: load i
+println("""
+define void @memref_collision($isz %x) {
+  %ptls = call {}*** @julia.ptls_states()
+  %ptls_i8 = bitcast {}*** %ptls to i8*
+  %v = call noalias {} addrspace(10)* @julia.gc_alloc_obj(i8* %ptls_i8, $isz 8, {} addrspace(10)* @tag)
+  %v_p = bitcast {} addrspace(10)* %v to $isz addrspace(10)*
+  store $isz %x, $isz addrspace(10)* %v_p
+  br i1 0, label %L1, label %L2
+
+L1:
+  %v1 = bitcast {} addrspace(10)* %v to {} addrspace(10)* addrspace(10)*
+  %v1_x = load {} addrspace(10)*, {} addrspace(10)* addrspace(10)* %v1
+  ret void
+
+L2:
+  %v2 = bitcast {} addrspace(10)* %v to $isz addrspace(10)*
+  %v2_x = load i64, i64 addrspace(10)* %v2
+  ret void
+}
+""")
+# CHECK-LABEL: }{{$}}