[Git][ghc/ghc][wip/andreask/bytecode_tagging] GHCi interpreter: Tag constructor closures when possible.
Andreas Klebinger (@AndreasK)
gitlab at gitlab.haskell.org
Tue Jun 18 14:35:58 UTC 2024
Andreas Klebinger pushed to branch wip/andreask/bytecode_tagging at Glasgow Haskell Compiler / GHC
Commits:
fa968687 by Andreas Klebinger at 2024-06-18T16:20:39+02:00
GHCi interpreter: Tag constructor closures when possible.
When evaluating PUSH_G try to tag the reference we are pushing if it's a
constructor or function. This is potentially helpful for performance and required to
fix #24870.
- - - - -
5 changed files:
- compiler/GHC/ByteCode/Instr.hs
- rts/Interpreter.c
- + testsuite/tests/th/should_compile/T24870/Def.hs
- + testsuite/tests/th/should_compile/T24870/Use.hs
- + testsuite/tests/th/should_compile/T24870/all.T
Changes:
=====================================
compiler/GHC/ByteCode/Instr.hs
=====================================
@@ -83,7 +83,7 @@ data BCInstr
| PUSH16_W !ByteOff
| PUSH32_W !ByteOff
- -- Push a ptr (these all map to PUSH_G really)
+ -- Push a (heap) ptr (these all map to PUSH_G really)
| PUSH_G Name
| PUSH_PRIMOP PrimOp
| PUSH_BCO (ProtoBCO Name)
=====================================
rts/Interpreter.c
=====================================
@@ -4,6 +4,30 @@
* Copyright (c) The GHC Team, 1994-2002.
* ---------------------------------------------------------------------------*/
+/*
+Note [CBV Functions and the interpreter]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When the byte code interpreter loads a reference to a value it often
+ends up as a non-tagged pointers *especially* if we already know a value
+is a certain constructor and therefore don't perform an eval on the reference.
+This causes friction with CBV functions which assume
+their value arguments are properly tagged by the caller.
+
+In order to ensure CBV functions still get passed tagged functions we have
+three options:
+a) Special case the interpreter behaviour into the tag inference analysis.
+ If we assume the interpreter can't properly tag value references the STG passes
+ would then wrap such calls in appropriate evals which are executed at runtime.
+ This would ensure tags by doing additional evals at runtime.
+b) When the interpreter pushes references for known constructors instead of
+ pushing the objects address add the tag to the value pushed. This is what
+ the NCG backends do.
+c) When the interpreter pushes a reference inspect the closure of the object
+ and apply the appropriate tag at runtime.
+
+For now we use approach c). Mostly because it's easiest to implement. We also don't
+tag functions as tag inference currently doesn't rely on those being properly tagged.
+*/
#include "rts/PosixSource.h"
#include "Rts.h"
@@ -292,6 +316,18 @@ STATIC_INLINE StgClosure *tagConstr(StgClosure *con) {
return TAG_CLOSURE(stg_min(TAG_MASK, 1 + GET_TAG(con)), con);
}
+// Compute the pointer tag for the function and tag the pointer;
+STATIC_INLINE StgClosure *tagFun(StgClosure *fun) {
+ StgHalfWord tag = GET_TAG(fun);
+ if(tag > TAG_MASK) { return fun; }
+ else {
+ return TAG_CLOSURE(tag, fun);
+ }
+
+
+}
+
+
static StgWord app_ptrs_itbl[] = {
(W_)&stg_ap_p_info,
(W_)&stg_ap_pp_info,
@@ -1306,7 +1342,52 @@ run_BCO:
case bci_PUSH_G: {
W_ o1 = BCO_GET_LARGE_ARG;
- SpW(-1) = BCO_PTR(o1);
+ StgClosure *tagged_obj = (StgClosure*) BCO_PTR(o1);
+
+ tag_push_g:
+ ASSERT(LOOKS_LIKE_CLOSURE_PTR((StgClosure*) tagged_obj));
+ // Here we make sure references we push are tagged.
+ // See Note [CBV Functions and the interpreter] in Info.hs
+
+ //Safe some memory reads if we already have a tag.
+ if(GET_CLOSURE_TAG(tagged_obj) == 0) {
+ StgClosure *obj = UNTAG_CLOSURE(tagged_obj);
+ switch ( get_itbl(obj)->type ) {
+ case IND:
+ case IND_STATIC:
+ {
+ tagged_obj = ACQUIRE_LOAD(&((StgInd*)obj)->indirectee);
+ goto tag_push_g;
+ }
+ case CONSTR:
+ case CONSTR_1_0:
+ case CONSTR_0_1:
+ case CONSTR_2_0:
+ case CONSTR_1_1:
+ case CONSTR_0_2:
+ case CONSTR_NOCAF:
+ // The value is already evaluated, so we can just return it. However,
+ // before we do, we MUST ensure that the pointer is tagged, because we
+ // might return to a native `case` expression, which assumes the returned
+ // pointer is tagged so it can use the tag to select an alternative.
+ tagged_obj = tagConstr(obj);
+ break;
+ case FUN:
+ case FUN_1_0:
+ case FUN_0_1:
+ case FUN_2_0:
+ case FUN_1_1:
+ case FUN_0_2:
+ case FUN_STATIC:
+ // Purely for performance since we already hit memory anyway.
+ tagged_obj = tagFun(obj);
+ break;
+ default:
+ break;
+ }
+ }
+
+ SpW(-1) = (W_) tagged_obj;
Sp_subW(1);
goto nextInsn;
}
=====================================
testsuite/tests/th/should_compile/T24870/Def.hs
=====================================
@@ -0,0 +1,9 @@
+{-# LANGUAGE TemplateHaskell #-}
+
+module SDef where
+
+{-# NOINLINE aValue #-}
+aValue = True
+
+{-# NOINLINE aStrictFunction #-}
+aStrictFunction !x = [| x |]
=====================================
testsuite/tests/th/should_compile/T24870/Use.hs
=====================================
@@ -0,0 +1,9 @@
+{-# LANGUAGE TemplateHaskell #-}
+
+module SUse where
+
+import qualified Language.Haskell.TH.Syntax as TH
+import SDef
+import GHC.Exts
+
+bar = $( inline aStrictFunction aValue )
=====================================
testsuite/tests/th/should_compile/T24870/all.T
=====================================
@@ -0,0 +1,6 @@
+# The interpreter must uphold tagging invariants, and failed to do so in #24870
+# We test this here by having the interpreter calls a strict worker function
+# with a reference to a value it constructed.
+# See also Note [CBV Functions and the interpreter]
+test('T24870', [extra_files(['Def.hs', 'Use.hs']), req_th],
+ multimod_compile, ['Def Use', '-dtag-inference-checks -v0'])
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/fa968687de81df65db846b15651ecd4e18eea3c6
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/fa968687de81df65db846b15651ecd4e18eea3c6
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