Make emitted egal code more loopy

The strategy here is to look at (data, padding) pairs and RLE
them into loops, so that repeated adjacent patterns use a loop
rather than getting unrolled. On the test case from #54109,
this makes compilation essentially instant, while also being
faster at runtime (turns out LLVM spends a massive amount of time
AND the answer is bad).

There's some obvious further enhancements possible here:
1. The `memcmp` constant is small. LLVM has a pass to inline these
   with better code. However, we don't have it turned on. We should
   consider vendoring it, though we may want to add some shorcutting
   to it to avoid having it iterate through each function.
2. This only does one level of sequence matching. It could be recursed
   to turn things into nested loops.

However, this solves the immediate issue, so hopefully it's a useful
start. Fixes #54109.

Author: Keno

src/codegen.cpp

@@ -3358,6 +3358,58 @@ static Value *emit_bitsunion_compare(jl_codectx_t &ctx, const jl_cgval_t &arg1,
     return phi;
 }
 
+struct egal_desc {
+    size_t offset;
+    size_t nrepeats;
+    size_t data_bytes;
+    size_t padding_bytes;
+};
+
+template <typename callback>
+static size_t emit_masked_bits_compare(callback &emit_desc, jl_datatype_t *aty, egal_desc &current_desc)
+{
+    // Memcmp, but with masked padding
+    size_t data_bytes = 0;
+    size_t padding_bytes = 0;
+    size_t nfields = jl_datatype_nfields(aty);
+    size_t total_size = jl_datatype_size(aty);
+    for (size_t i = 0; i < nfields; ++i) {
+        size_t offset = jl_field_offset(aty, i);
+        size_t fend = i == nfields - 1 ? total_size : jl_field_offset(aty, i + 1);
+        size_t fsz = jl_field_size(aty, i);
+        jl_datatype_t *fty = (jl_datatype_t*)jl_field_type(aty, i);
+        if (jl_field_isptr(aty, i) || !fty->layout->flags.haspadding) {
+            // The field has no internal padding
+            data_bytes += fsz;
+            if (offset + fsz == fend) {
+                // The field has no padding after. Merge this into the current
+                // comparison range and go to next field.
+            } else {
+                padding_bytes = fend - offset - fsz;
+                // Found padding. Either merge this into the current comparison
+                // range, or emit the old one and start a new one.
+                if (current_desc.data_bytes == data_bytes &&
+                        current_desc.padding_bytes == padding_bytes) {
+                    // Same as the previous range, just note that down, so we
+                    // emit this as a loop.
+                    current_desc.nrepeats += 1;
+                } else {
+                    if (current_desc.nrepeats != 0)
+                        emit_desc(current_desc);
+                    current_desc.nrepeats = 1;
+                    current_desc.data_bytes = data_bytes;
+                    current_desc.padding_bytes = padding_bytes;
+                }
+                data_bytes = 0;
+            }
+        } else {
+            // The field may have internal padding. Recurse this.
+            data_bytes += emit_masked_bits_compare(emit_desc, fty, current_desc);
+        }
+    }
+    return data_bytes;
+}
+
 static Value *emit_bits_compare(jl_codectx_t &ctx, jl_cgval_t arg1, jl_cgval_t arg2)
 {
     ++EmittedBitsCompares;
@@ -3396,7 +3448,7 @@ static Value *emit_bits_compare(jl_codectx_t &ctx, jl_cgval_t arg1, jl_cgval_t a
     if (at->isAggregateType()) { // Struct or Array
         jl_datatype_t *sty = (jl_datatype_t*)arg1.typ;
         size_t sz = jl_datatype_size(sty);
-        if (sz > 512 && !sty->layout->flags.haspadding) {
+        if (sz > 512 && !sty->layout->flags.haspadding && sty->layout->flags.isbitsegal) {
             Value *varg1 = arg1.ispointer() ? data_pointer(ctx, arg1) :
                 value_to_pointer(ctx, arg1).V;
             Value *varg2 = arg2.ispointer() ? data_pointer(ctx, arg2) :
@@ -3433,6 +3485,89 @@ static Value *emit_bits_compare(jl_codectx_t &ctx, jl_cgval_t arg1, jl_cgval_t a
             }
             return ctx.builder.CreateICmpEQ(answer, ConstantInt::get(getInt32Ty(ctx.builder.getContext()), 0));
         }
+        else if (sz > 512 && jl_struct_try_layout(sty) && sty->layout->flags.isbitsegal) {
+            Type *TInt8 = getInt8Ty(ctx.builder.getContext());
+            Type *TpInt8 = getInt8PtrTy(ctx.builder.getContext());
+            Type *TInt1 = getInt1Ty(ctx.builder.getContext());
+            Value *varg1 = arg1.ispointer() ? data_pointer(ctx, arg1) :
+                value_to_pointer(ctx, arg1).V;
+            Value *varg2 = arg2.ispointer() ? data_pointer(ctx, arg2) :
+                value_to_pointer(ctx, arg2).V;
+            varg1 = emit_pointer_from_objref(ctx, varg1);
+            varg2 = emit_pointer_from_objref(ctx, varg2);
+            varg1 = emit_bitcast(ctx, varg1, TpInt8);
+            varg2 = emit_bitcast(ctx, varg2, TpInt8);
+
+            Value *answer = nullptr;
+            auto emit_desc = [&](egal_desc desc) {
+                Value *ptr1 = varg1;
+                Value *ptr2 = varg2;
+                if (desc.offset != 0) {
+                    ptr1 = ctx.builder.CreateConstInBoundsGEP1_32(TInt8, ptr1, desc.offset);
+                    ptr2 = ctx.builder.CreateConstInBoundsGEP1_32(TInt8, ptr2, desc.offset);
+                }
+
+                Value *new_ptr1 = ptr1;
+                Value *endptr1 = nullptr;
+                BasicBlock *postBB = nullptr;
+                BasicBlock *loopBB = nullptr;
+                PHINode *answerphi = nullptr;
+                if (desc.nrepeats != 1) {
+                    // Set up loop
+                    endptr1 = ctx.builder.CreateConstInBoundsGEP1_32(TInt8, ptr1, desc.nrepeats * (desc.data_bytes + desc.padding_bytes));;
+
+                    BasicBlock *currBB = ctx.builder.GetInsertBlock();
+                    loopBB = BasicBlock::Create(ctx.builder.getContext(), "egal_loop", ctx.f);
+                    postBB = BasicBlock::Create(ctx.builder.getContext(), "post", ctx.f);
+                    ctx.builder.CreateBr(loopBB);
+
+                    ctx.builder.SetInsertPoint(loopBB);
+                    answerphi = ctx.builder.CreatePHI(TInt1, 2);
+                    answerphi->addIncoming(answer ? answer : ConstantInt::get(TInt1, 1), currBB);
+                    answer = answerphi;
+
+                    PHINode *itr1 = ctx.builder.CreatePHI(ptr1->getType(), 2);
+                    PHINode *itr2 = ctx.builder.CreatePHI(ptr2->getType(), 2);
+
+                    new_ptr1 = ctx.builder.CreateConstInBoundsGEP1_32(TInt8, itr1, desc.data_bytes + desc.padding_bytes);
+                    itr1->addIncoming(ptr1, currBB);
+                    itr1->addIncoming(new_ptr1, loopBB);
+
+                    Value *new_ptr2 = ctx.builder.CreateConstInBoundsGEP1_32(TInt8, itr2, desc.data_bytes + desc.padding_bytes);
+                    itr2->addIncoming(ptr2, currBB);
+                    itr2->addIncoming(new_ptr2, loopBB);
+
+                    ptr1 = itr1;
+                    ptr2 = itr2;
+                }
+
+                // Emit memcmp. TODO: LLVM has a pass to expand this for additional
+                // performance.
+                Value *this_answer = ctx.builder.CreateCall(prepare_call(memcmp_func),
+                    { ptr1,
+                      ptr2,
+                      ConstantInt::get(ctx.types().T_size, desc.data_bytes) });
+                this_answer = ctx.builder.CreateICmpEQ(this_answer, ConstantInt::get(getInt32Ty(ctx.builder.getContext()), 0));
+                answer = answer ? ctx.builder.CreateAnd(answer, this_answer) : this_answer;
+                if (endptr1) {
+                    answerphi->addIncoming(answer, loopBB);
+                    Value *loopend = ctx.builder.CreateICmpEQ(new_ptr1, endptr1);
+                    ctx.builder.CreateCondBr(loopend, postBB, loopBB);
+                    ctx.builder.SetInsertPoint(postBB);
+                }
+            };
+            egal_desc current_desc = {0};
+            size_t trailing_data_bytes = emit_masked_bits_compare(emit_desc, sty, current_desc);
+            assert(current_desc.nrepeats != 0);
+            emit_desc(current_desc);
+            if (trailing_data_bytes != 0) {
+                current_desc.nrepeats = 1;
+                current_desc.data_bytes = trailing_data_bytes;
+                current_desc.padding_bytes = 0;
+                emit_desc(current_desc);
+            }
+            return answer;
+        }
         else {
             jl_svec_t *types = sty->types;
             Value *answer = ConstantInt::get(getInt1Ty(ctx.builder.getContext()), 1);

src/datatype.c

@@ -180,6 +180,7 @@ static jl_datatype_layout_t *jl_get_layout(uint32_t sz,
                                            uint32_t npointers,
                                            uint32_t alignment,
                                            int haspadding,
+                                           int isbitsegal,
                                            int arrayelem,
                                            jl_fielddesc32_t desc[],
                                            uint32_t pointers[]) JL_NOTSAFEPOINT
@@ -226,6 +227,7 @@ static jl_datatype_layout_t *jl_get_layout(uint32_t sz,
     flddesc->nfields = nfields;
     flddesc->alignment = alignment;
     flddesc->flags.haspadding = haspadding;
+    flddesc->flags.isbitscomparable = isbitscomparable;
     flddesc->flags.fielddesc_type = fielddesc_type;
     flddesc->flags.arrayelem_isboxed = arrayelem == 1;
     flddesc->flags.arrayelem_isunion = arrayelem == 2;
@@ -504,6 +506,7 @@ void jl_get_genericmemory_layout(jl_datatype_t *st)
     int isunboxed = jl_islayout_inline(eltype, &elsz, &al) && (kind != (jl_value_t*)jl_atomic_sym || jl_is_datatype(eltype));
     int isunion = isunboxed && jl_is_uniontype(eltype);
     int haspadding = 1; // we may want to eventually actually compute this more precisely
+    int isbitsegal = 0;
     int nfields = 0; // aka jl_is_layout_opaque
     int npointers = 1;
     int zi;
@@ -562,7 +565,7 @@ void jl_get_genericmemory_layout(jl_datatype_t *st)
     else
         arrayelem = 0;
     assert(!st->layout);
-    st->layout = jl_get_layout(elsz, nfields, npointers, al, haspadding, arrayelem, NULL, pointers);
+    st->layout = jl_get_layout(elsz, nfields, npointers, al, haspadding, isbitsegal, arrayelem, NULL, pointers);
     st->zeroinit = zi;
     //st->has_concrete_subtype = 1;
     //st->isbitstype = 0;
@@ -673,6 +676,7 @@ void jl_compute_field_offsets(jl_datatype_t *st)
         size_t alignm = 1;
         int zeroinit = 0;
         int haspadding = 0;
+        int isbitsegal = 1;
         int homogeneous = 1;
         int needlock = 0;
         uint32_t npointers = 0;
@@ -687,19 +691,30 @@ void jl_compute_field_offsets(jl_datatype_t *st)
                     throw_ovf(should_malloc, desc, st, fsz);
                 desc[i].isptr = 0;
                 if (jl_is_uniontype(fld)) {
-                    haspadding = 1;
                     fsz += 1; // selector byte
                     zeroinit = 1;
+                    // TODO: Some unions could be bits comparable.
+                    isbitsegal = 0;
                 }
                 else {
                     uint32_t fld_npointers = ((jl_datatype_t*)fld)->layout->npointers;
                     if (((jl_datatype_t*)fld)->layout->flags.haspadding)
                         haspadding = 1;
+                    if (!((jl_datatype_t*)fld)->layout->flags.isbitsegal)
+                        isbitsegal = 0;
                     if (i >= nfields - st->name->n_uninitialized && fld_npointers &&
                         fld_npointers * sizeof(void*) != fsz) {
-                        // field may be undef (may be uninitialized and contains pointer),
-                        // and contains non-pointer fields of non-zero sizes.
-                        haspadding = 1;
+                        // For field types that contain pointers, we allow inlinealloc
+                        // as long as the field type itself is always fully initialized.
+                        // In such a case, we use the first pointer in the inlined field
+                        // as the #undef marker (if it is zero, we treat the whole inline
+                        // struct as #undef). However, we do not zero-initialize the whole
+                        // struct, so the non-pointer parts of the inline allocation may
+                        // be arbitrary, but still need to compare egal (because all #undef)
+                        // representations are egal. Because of this, we cannot bitscompare
+                        // them.
+                        // TODO: Consider zero-initializing the whole struct.
+                        isbitsegal = 0;
                     }
                     if (!zeroinit)
                         zeroinit = ((jl_datatype_t*)fld)->zeroinit;
@@ -715,8 +730,7 @@ void jl_compute_field_offsets(jl_datatype_t *st)
                 zeroinit = 1;
                 npointers++;
                 if (!jl_pointer_egal(fld)) {
-                    // this somewhat poorly named flag says whether some of the bits can be non-unique
-                    haspadding = 1;
+                    isbitsegal = 0;
                 }
             }
             if (isatomic && fsz > MAX_ATOMIC_SIZE)
@@ -777,7 +791,7 @@ void jl_compute_field_offsets(jl_datatype_t *st)
             }
         }
         assert(ptr_i == npointers);
-        st->layout = jl_get_layout(sz, nfields, npointers, alignm, haspadding, 0, desc, pointers);
+        st->layout = jl_get_layout(sz, nfields, npointers, alignm, haspadding, isbitsegal, 0, desc, pointers);
         if (should_malloc) {
             free(desc);
             if (npointers)
@@ -931,7 +945,7 @@ JL_DLLEXPORT jl_datatype_t *jl_new_primitivetype(jl_value_t *name, jl_module_t *
     bt->ismutationfree = 1;
     bt->isidentityfree = 1;
     bt->isbitstype = (parameters == jl_emptysvec);
-    bt->layout = jl_get_layout(nbytes, 0, 0, alignm, 0, 0, NULL, NULL);
+    bt->layout = jl_get_layout(nbytes, 0, 0, alignm, 0, 1, 0, NULL, NULL);
     bt->instance = NULL;
     return bt;
 }

src/julia.h

@@ -574,7 +574,10 @@ typedef struct {
         // metadata bit only for GenericMemory eltype layout
         uint16_t arrayelem_isboxed : 1;
         uint16_t arrayelem_isunion : 1;
-        uint16_t padding : 11;
+        // If set, this type's egality can be determined entirely by comparing
+        // the non-padding bits of this datatype.
+        uint16_t isbitsegal : 1;
+        uint16_t padding : 10;
     } flags;
     // union {
     //     jl_fielddesc8_t field8[nfields];

test/compiler/codegen.jl

@@ -873,3 +873,54 @@ if Sys.ARCH === :x86_64
         end
     end
 end
+
+# #54109 - Excessive LLVM time for egal
+struct DefaultOr54109{T}
+    x::T
+    default::Bool
+end
+
+@eval struct Torture1_54109
+    $((Expr(:(::), Symbol("x$i"), DefaultOr54109{Float64}) for i = 1:897)...)
+end
+Torture1_54109() = Torture1_54109((DefaultOr54109(1.0, false) for i = 1:897)...)
+
+@eval struct Torture2_54109
+    $((Expr(:(::), Symbol("x$i"), DefaultOr54109{Float64}) for i = 1:400)...)
+    $((Expr(:(::), Symbol("x$(i+400)"), DefaultOr54109{Int16}) for i = 1:400)...)
+end
+Torture2_54109() = Torture2_54109((DefaultOr54109(1.0, false) for i = 1:400)..., (DefaultOr54109(Int16(1), false) for i = 1:400)...)
+
+@noinline egal_any54109(x, @nospecialize(y::Any)) = x === Base.compilerbarrier(:type, y)
+
+let ir1 = get_llvm(egal_any54109, Tuple{Torture1_54109, Any}),
+    ir2 = get_llvm(egal_any54109, Tuple{Torture2_54109, Any})
+
+    # We can't really do timing on CI, so instead, let's look at the length of
+    # the optimized IR. The original version had tens of thousands of lines and
+    # was slower, so just check here that we only have < 500 lines. If somebody,
+    # implements a better comparison that's larger than that, just re-benchmark
+    # this and adjust the threshold.
+
+    @test count(==('\n'), ir1) < 500
+    @test count(==('\n'), ir2) < 500
+end
+
+## Regression test for egal of a struct of this size without padding, but with
+## non-bitsegal, to make sure that it doesn't accidentally go down the accelerated
+## path.
+@eval struct BigStructAnyInt
+    $((Expr(:(::), Symbol("x$i"), Tuple{Any, Int}) for i = 1:33)...)
+end
+BigStructAnyInt() = BigStructAnyInt(((Union{Base.inferencebarrier(Float64), Int}, i) for i = 1:33)...)
+@test egal_any54109(BigStructAnyInt(), BigStructAnyInt())
+
+## For completeness, also test correctness, since we don't have a lot of
+## large-struct tests.
+
+# The two allocations of the same struct will likely have different padding,
+# we want to make sure we find them egal anyway - a naive memcmp would
+# accidentally look at it.
+@test egal_any54109(Torture1_54109(), Torture1_54109())
+@test egal_any54109(Torture2_54109(), Torture2_54109())
+@test !egal_any54109(Torture1_54109(), Torture1_54109((DefaultOr54109(2.0, false) for i = 1:897)...))