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[mono][wasm] Rework the handling of GC references in AOTed code. #59352

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Sep 21, 2021
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[mono][wasm] Rework the handling of GC references in AOTed code. #59352

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6 changes: 1 addition & 5 deletions src/mono/mono/mini/ir-emit.h
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Original file line number Diff line number Diff line change
Expand Up @@ -61,11 +61,7 @@ alloc_ireg_ref (MonoCompile *cfg)
mono_mark_vreg_as_ref (cfg, vreg);

#ifdef TARGET_WASM
/*
* For GC stack scanning to work, have to spill all reference variables to the stack.
*/
MonoInst *ins = mono_compile_create_var_for_vreg (cfg, m_class_get_byval_arg (mono_get_object_class ()), OP_LOCAL, vreg);
ins->flags |= MONO_INST_VOLATILE;
mono_mark_vreg_as_ref (cfg, vreg);
#endif

return vreg;
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72 changes: 60 additions & 12 deletions src/mono/mono/mini/mini-llvm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -193,6 +193,8 @@ typedef struct {
char *method_name;
GHashTable *jit_callees;
LLVMValueRef long_bb_break_var;
int *gc_var_indexes;
LLVMValueRef gc_pin_area;
} EmitContext;

typedef struct {
Expand Down Expand Up @@ -3732,6 +3734,18 @@ emit_unbox_tramp (EmitContext *ctx, const char *method_name, LLVMTypeRef method_
LLVMDisposeBuilder (builder);
}

#ifdef TARGET_WASM
static void
emit_gc_pin (EmitContext *ctx, LLVMBuilderRef builder, int vreg)
{
LLVMValueRef index0 = LLVMConstInt (LLVMInt32Type (), 0, FALSE);
LLVMValueRef index1 = LLVMConstInt (LLVMInt32Type (), ctx->gc_var_indexes [vreg] - 1, FALSE);
LLVMValueRef indexes [] = { index0, index1 };
LLVMValueRef addr = LLVMBuildGEP (builder, ctx->gc_pin_area, indexes, 2, "");
mono_llvm_build_store (builder, convert (ctx, ctx->values [vreg], IntPtrType ()), addr, TRUE, LLVM_BARRIER_NONE);
}
#endif

/*
* emit_entry_bb:
*
Expand All @@ -3752,6 +3766,27 @@ emit_entry_bb (EmitContext *ctx, LLVMBuilderRef builder)

ctx->alloca_builder = create_builder (ctx);

#ifdef TARGET_WASM
/*
* For GC stack scanning to work, allocate an area on the stack and store
* every ref vreg into it after its written. Because the stack is scanned
* conservatively, the objects will be pinned, so the vregs can directly
* reference the objects, there is no need to load them from the stack
* on every access.
*/
ctx->gc_var_indexes = g_new0 (int, cfg->next_vreg);
int ngc_vars = 0;
for (i = 0; i < cfg->next_vreg; ++i) {
if (vreg_is_ref (cfg, i)) {
ctx->gc_var_indexes [i] = ngc_vars + 1;
ngc_vars ++;
}
}

// FIXME: Count only live vregs
ctx->gc_pin_area = build_alloca_llvm_type_name (ctx, LLVMArrayType (IntPtrType (), ngc_vars), 0, "gc_pin");
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#endif

/*
* Handle indirect/volatile variables by allocating memory for them
* using 'alloca', and storing their address in a temporary.
Expand All @@ -3763,13 +3798,6 @@ emit_entry_bb (EmitContext *ctx, LLVMBuilderRef builder)
if ((var->opcode == OP_GSHAREDVT_LOCAL || var->opcode == OP_GSHAREDVT_ARG_REGOFFSET))
continue;

#ifdef TARGET_WASM
// For GC stack scanning to work, have to spill all reference variables to the stack
// Some ref variables have type intptr
if (ctx->has_safepoints && (MONO_TYPE_IS_REFERENCE (var->inst_vtype) || var->inst_vtype->type == MONO_TYPE_I) && var != ctx->cfg->rgctx_var)
var->flags |= MONO_INST_INDIRECT;
#endif

if (var->flags & (MONO_INST_VOLATILE|MONO_INST_INDIRECT) || (mini_type_is_vtype (var->inst_vtype) && !MONO_CLASS_IS_SIMD (ctx->cfg, var->klass))) {
vtype = type_to_llvm_type (ctx, var->inst_vtype);
if (!ctx_ok (ctx))
Expand Down Expand Up @@ -3966,6 +3994,19 @@ emit_entry_bb (EmitContext *ctx, LLVMBuilderRef builder)
}
}

#ifdef TARGET_WASM
/*
* Store ref arguments to the pin area.
* FIXME: This might not be needed, since the caller already does it ?
*/
for (i = 0; i < cfg->num_varinfo; ++i) {
MonoInst *var = cfg->varinfo [i];

if (var->opcode == OP_ARG && vreg_is_ref (cfg, var->dreg) && ctx->values [var->dreg])
emit_gc_pin (ctx, builder, var->dreg);
}
#endif

/* Initialize the method if needed */
if (cfg->compile_aot) {
/* Emit a location for the initialization code */
Expand Down Expand Up @@ -10998,9 +11039,15 @@ process_bb (EmitContext *ctx, MonoBasicBlock *bb)
values [ins->dreg] = convert (ctx, values [ins->dreg], ctx->vreg_types [ins->dreg]);
}

/* Add stores for volatile variables */
if (!skip_volatile_store && spec [MONO_INST_DEST] != ' ' && spec [MONO_INST_DEST] != 'v' && !MONO_IS_STORE_MEMBASE (ins))
emit_volatile_store (ctx, ins->dreg);
/* Add stores for volatile/ref variables */
if (spec [MONO_INST_DEST] != ' ' && spec [MONO_INST_DEST] != 'v' && !MONO_IS_STORE_MEMBASE (ins)) {
if (!skip_volatile_store)
emit_volatile_store (ctx, ins->dreg);
#ifdef TARGET_WASM
if (vreg_is_ref (cfg, ins->dreg) && ctx->values [ins->dreg])
emit_gc_pin (ctx, builder, ins->dreg);
#endif
}
}

if (!ctx_ok (ctx))
Expand Down Expand Up @@ -11152,6 +11199,7 @@ free_ctx (EmitContext *ctx)
g_free (ctx->vreg_cli_types);
g_free (ctx->is_dead);
g_free (ctx->unreachable);
g_free (ctx->gc_var_indexes);
g_ptr_array_free (ctx->phi_values, TRUE);
g_free (ctx->bblocks);
g_hash_table_destroy (ctx->region_to_handler);
Expand Down Expand Up @@ -12015,12 +12063,12 @@ emit_method_inner (EmitContext *ctx)
g_free (name);
}

/*
#if 0
int err = LLVMVerifyFunction (ctx->lmethod, LLVMPrintMessageAction);
if (err != 0)
LLVMDumpValue (ctx->lmethod);
g_assert (err == 0);
*/
#endif
} else {
//LLVMVerifyFunction (method, 0);
llvm_jit_finalize_method (ctx);
Expand Down
5 changes: 5 additions & 0 deletions src/mono/mono/mini/mini.c
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Original file line number Diff line number Diff line change
Expand Up @@ -669,6 +669,11 @@ mono_compile_create_var_for_vreg (MonoCompile *cfg, MonoType *type, int opcode,
}
}
}

#ifdef TARGET_WASM
if (mini_type_is_reference (type))
mono_mark_vreg_as_ref (cfg, vreg);
#endif

cfg->varinfo [num] = inst;

Expand Down