On 14/12/2023 10:32, Peter Eisentraut wrote:
I notice that the existing comments point out that the size argument
should be a compile-time constant, but that is no longer the case for
ExtensibleNode(). Also, newNode() is the only caller of palloc0fast(),
which also points out that the size argument should be a compile-time
constant, and palloc0fast() is the only caller of MemSetTest(). I can
see how an older compiler might have gotten too confused by all that.
But if we think that compilers are now smart enough, maybe we can unwind
this whole stack a bit more? Maybe we don't need MemSetTest() and/or
palloc0fast() and/or newNode() at all?
Good point. Looking closer, modern compilers will actually turn the
MemSetLoop() in MemoryContextAllocZeroAligned() into a call to memset()
anyway! Funny. That is true for recent versions of gcc, clang, and MSVC.
Here's a link to a godbolt snippet to play with this:
https://godbolt.org/z/9b89P3c8x (full link at [0]).
Yeah, +1 on removing all that, including MemoryContextAllocZeroAligned.
It's not doing any good as it is, as it gets compiled to be identical to
MemoryContextAllocZero. (There are small differences depending compiler
and version, but e.g. on gcc 13.2, the code generated for
MemoryContextAllocZero() is actually smaller even though both call memset())
Another approach would be to add more hints to
MemoryContextAllocZeroAligned to dissuade the compiler from turning the
loop into a memset() call. If you add an "if (size > 1024) abort" there,
then gcc 13 doesn't optimize into a memset() call, but clang still does.
Some micro-benchmarks on that would be nice.
But given that the compiler has been doing this optimization for a while
and we haven't noticed, I think memset() should be considered the status
quo, and converting it to a loop again should be considered a new
optimization.
Also, replacing MemoryContextAllocZeroAligned(CurrentMemoryContext,
size) with palloc0(size) has one fewer argument and the assembly code of
the call has one fewer instruction. That's something too.
[0]
https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGe1wAyeAyYAHI%2BAEaYxBIa0gAOqAqETgwe3r56icmOAkEh4SxRMVxxtpj2uQxCBEzEBOk%2Bflzllak1dQT5YZHRsdIKtfWNmS2Dnd2Fxf0AlLaoXsTI7BzmAMzByN5YANQma26D6BGongB0CPvYJhoAguub25h7B0e0eBEXVzf3ZhsMWy8u32hwI6CwVC%2Ba2udweAKeL1BxGCwChMPudwA9AAqHZUYioFg7ZAXHbYzE/B5UHYAfRpAHFQnI3HSXtc1gARHZrSl/CHBZ7vADWFQAnhBVDMbgBOOkRLx0RwMGmYVTxTAOKCSnZgMD7LkaUg7LhS2F8zA0EI7LwMYViiUzWk0%2BWK4IqtUaghax26/U7Q3%2B02/NYVJS8tb8q122jiyUy706vWcwPhyPPG3R2OOhO%2B5MaIPrVx4KiUs0Ri0CnYAWSYqlutFoqGQQjwAC9MPGIC3246NKo1lRB0OqI6ccadsA8MAmBFRQRngBacfk1MVq31xvNtuYACSCgAamI8OgIMke53u5hs2er4i/TW6w2m5eC3dBsQvA5q5gWCRRR5BFVAgOSYWp9isO4CFFdUIR2d9PwIb9f2If9ZCAkDajJKsfz/AD51UAhwJ%2BKCYItODtxpRDLyIyDoMwWD4K/bDkNQwCCI8G152IBQkNwtD2IWQDuJojFbhI%2BiyIAN1QY8dggbFmL/DoCAUZRkUEAAxG1kGzRSULwoDiX4ggjWk2TsXiJgFEGBAP1IUtpRlJyNEclznNcjydiMhhBmJBA6jJcYVJpd8UXs2E3Mijz3JilyvJOTwdnidSCEo1AQrBaJiClNYINE8SGIID8mJw/TjOwggEAwBRKQAdjymUzPQGUvLJeSxE3XTStY/DELQNiTJ2S9yPPXKHLHUQGx2Sq8B4/FMEwGl4h2YJiSs54FEJTABGedY0zADg5uIBaIEdFcIulJr3Pk%2BbFvibMmrJRJgi4nKGpcq64q87F5OOjqmwemT0CemShKNYabzehzPo837MCUAgupYgyCO83qoYumH42xLB6HnGl%2BvRuS9J6wzCaAjGMWlEmUcQ%2BTgEwVLkAQG0hQJ4zAfM56hMpmVqKi%2BnGYJlmGDZhRLOWTngYs0HXpEmUEtoa7sVmlUWHiKCkb4ga0Yp%2BWPqB5WgoULWyp18mCPCqnYuir6SeU1SUq0gEkpSqhtNMoGnqsmy7Icm33K8mnjI4oSeOxIhaloBQrYDqKWp2RXXZetKMqwYhsvlv5i1gqtsCrAB5AAlABNGk3AL0IABVsAADSr8uAAlsDcABpHdQnpaHDYF7FmY1IVTdJ1GLcRrOQwYfAS1hWquWDgaKqq9AarGiKxLowrisQ%2Bfeowpg6ryrzMUxJLgBpBgMEW0CiogGd3yYBxTQ8sdKuiZ46meC%2BdgvrAeNQakZo8XEmSCka8j6gOlKES%2BVcmDAHcuJGittpRjmPIwRwNB4Y7FVA/RCQpgjA3/rrVG50qYvwQPDZ4BAADuqA8R4AqMvHYH8koGGWMDchx1pq0JYMEPAPD2xMPeMABgbBBA7CoVZWoDMQAgIcordys0i4UMIqvZ%2BuIq4vC5F/cWD935PjYeRAEgpJE7GOgjWRF15FRX%2BlQncaRkTCE9KkcCOwxw2KSv9ZAK0GDEgcXgSacEnECAsVTfmHk2AsBpJ40C9FEEv2IA/IUOwIl/kEZuGJwN%2BDEGmggWaRDEIkJlP1XyO8gKL2qmSNglVqpxNxJJPA9QvBiDxNpKo00Zz0BCTKUpqNLLHUEIgmUY4mQBACCtakX8%2BloLkkQeI9BJIVHyWdUBVMemIRoNxAgzM6DNVUWOchTACHUneKLeiOx3i%2BUIds2g6B%2BldJcms12mBJLXN2RBZBuJkrPKcF4HirydiEIUKwZ4UyxGFIed1Wm38gKvNqdCkeuSbkAupECtgHj%2BkFJWUUgQvlmYBWxMwNggyooTWMt/YFcltC/PWSQHYWB5TAEnEYZZDlil9X8tklWWABmqPcqSnWO4uTFiYQwUUlLqV4lpfSrwjKUQsruOAtZbgOoRESWSMxQtkARBXu8scFzEKEI1QQTEuNGbPEmrQVVyAhThyxbPXiZtd6gX3qvUSq5LTPGYkIRmARTjxFPEMQakkxBGnoAwR0JgACsbgHLoFoVGmNF0TD1T2NG/2LlGxGDJLScY9RNFyUzcAMk15A1vVTYm62GaBBFtxBlVAy0/QQELcWuSEA8WcsdBlToexLByRfAWxgMwy0Jv9mmpNLkqGIueBALteaQQ5tmRGsdlaPLYlnYRSw1h80aBcSOpN9rJ10GnfmUsolHrYh%2BGsjcTYABa0RUD1inCEE8jzR7g23CNK8B9u7HgTuqxm%2BtSGYluNZaIXo1l7kPO8E8o8h362QcB0D9QoEEDsW4Pxk1vUOFSG2jmgGZTCogLqa9W52yQaPCeSGr5pReRnCQL0vMLqjwXFcRRyj81UDEGGV1DljqIT9Mxq4VSl4KBY9CTxuGBpGkhvBwjGY8AihjBAPjmi/QjICHB/d70aNMJOPUU68Hk0clPUMzE%2B5bgBHpEXDuHIaR5yrMoAuBcAg0gswEAubhJO9SNHx6T25GNAa9T6v1ynGZGgDDJ11rVkk/gRqFwaEX/P4ZcnxxYPi%2BPy1nj8DgcxaCcEjbwPwHAtCkFQJwGNm7e2bUWMsHtaweCkAIJoHLcwhQgEjYaPLHBJCFea6VzgvAFAgENE14rOXSBwFgEgNA6sj1kAoLh2b9AYjAC4GsMwfBFTRCGxACIfWIjBDqKKTgDWDvMBQgXCI2hPQnd4DN0RBAC62mO2N0g0rgDKobEN7gvAsAsEMMAcQr38DHWwws77JXVQai8POW75BAJdZK%2B8CICSypYD60VPht25hUAMMAA89CqEF3VEVhr/BBAiDEOwKQMhBCKBUOoV7ugWgGCMCgaw1h9AfCG5AOY9aqjfYXAXMwvBUALIzqgnnp1WhBL8BAVwIxmikECAKKYfQWjZBSAIRXWQkha4YJMXoJQZfYYEB0YYngmh6DsLL83XRVdG%2Bt4GnXYxA2G6KOruY1WljU9y/l3rr2yscB2KoAAHAANgXOHyQE5kBeLW2cMwclcCEFpesE0vBRtaCHaQNrHX9CcB66QIrJWg%2BDeG415rOeusi%2BL31svlexs5/F8kZwkggA%3D%3D
--
Heikki Linnakangas
Neon (https://neon.tech)
From 15e93b1b9fdaf19dd55f943ab4f567b5feed2961 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakan...@iki.fi>
Date: Thu, 14 Dec 2023 14:44:10 +0100
Subject: [PATCH v2 1/1] Simplify newNode() by removing now-unnecessary fast
paths and special cases
- Convert newNode() to a static inline function, to make it simpler. We
didn't require static inline support from compiler when this was
originally written, but now we do.
- Remove MemoryContextAllocZeroAligned(). It was supposed to be a
faster version of MemoryContextAllocZero(), but modern compilers turn
the MemSetLoop() into a call to memset() anyway, making it identical to
MemoryContextAllocZero(). That was the only user of MemSetTest,
MemSetLoop, and palloc0fast(), so remove all those too.
Discussion: XXX
Reviewed-by: Peter Eisentraut
---
src/backend/nodes/Makefile | 1 -
src/backend/nodes/meson.build | 1 -
src/backend/nodes/nodes.c | 31 ---------------------------
src/backend/utils/mmgr/mcxt.c | 38 ---------------------------------
src/include/c.h | 24 ---------------------
src/include/nodes/nodes.h | 40 +++++++++--------------------------
src/include/utils/palloc.h | 14 ------------
7 files changed, 10 insertions(+), 139 deletions(-)
delete mode 100644 src/backend/nodes/nodes.c
diff --git a/src/backend/nodes/Makefile b/src/backend/nodes/Makefile
index ebbe9052cb7..66bbad8e6e0 100644
--- a/src/backend/nodes/Makefile
+++ b/src/backend/nodes/Makefile
@@ -23,7 +23,6 @@ OBJS = \
makefuncs.o \
multibitmapset.o \
nodeFuncs.o \
- nodes.o \
outfuncs.o \
params.o \
print.o \
diff --git a/src/backend/nodes/meson.build b/src/backend/nodes/meson.build
index 31467a12d3b..1efbf2c11ca 100644
--- a/src/backend/nodes/meson.build
+++ b/src/backend/nodes/meson.build
@@ -7,7 +7,6 @@ backend_sources += files(
'makefuncs.c',
'multibitmapset.c',
'nodeFuncs.c',
- 'nodes.c',
'params.c',
'print.c',
'read.c',
diff --git a/src/backend/nodes/nodes.c b/src/backend/nodes/nodes.c
deleted file mode 100644
index 1913a4bdf7d..00000000000
--- a/src/backend/nodes/nodes.c
+++ /dev/null
@@ -1,31 +0,0 @@
-/*-------------------------------------------------------------------------
- *
- * nodes.c
- * support code for nodes (now that we have removed the home-brew
- * inheritance system, our support code for nodes is much simpler)
- *
- * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
- * Portions Copyright (c) 1994, Regents of the University of California
- *
- *
- * IDENTIFICATION
- * src/backend/nodes/nodes.c
- *
- * HISTORY
- * Andrew Yu Oct 20, 1994 file creation
- *
- *-------------------------------------------------------------------------
- */
-#include "postgres.h"
-
-#include "nodes/nodes.h"
-
-/*
- * Support for newNode() macro
- *
- * In a GCC build there is no need for the global variable newNodeMacroHolder.
- * However, we create it anyway, to support the case of a non-GCC-built
- * loadable module being loaded into a GCC-built backend.
- */
-
-Node *newNodeMacroHolder;
diff --git a/src/backend/utils/mmgr/mcxt.c b/src/backend/utils/mmgr/mcxt.c
index 9fc83f11f6f..4b30fcaebd0 100644
--- a/src/backend/utils/mmgr/mcxt.c
+++ b/src/backend/utils/mmgr/mcxt.c
@@ -1091,44 +1091,6 @@ MemoryContextAllocZero(MemoryContext context, Size size)
return ret;
}
-/*
- * MemoryContextAllocZeroAligned
- * MemoryContextAllocZero where length is suitable for MemSetLoop
- *
- * This might seem overly specialized, but it's not because newNode()
- * is so often called with compile-time-constant sizes.
- */
-void *
-MemoryContextAllocZeroAligned(MemoryContext context, Size size)
-{
- void *ret;
-
- Assert(MemoryContextIsValid(context));
- AssertNotInCriticalSection(context);
-
- if (!AllocSizeIsValid(size))
- elog(ERROR, "invalid memory alloc request size %zu", size);
-
- context->isReset = false;
-
- ret = context->methods->alloc(context, size);
- if (unlikely(ret == NULL))
- {
- MemoryContextStats(TopMemoryContext);
- ereport(ERROR,
- (errcode(ERRCODE_OUT_OF_MEMORY),
- errmsg("out of memory"),
- errdetail("Failed on request of size %zu in memory context \"%s\".",
- size, context->name)));
- }
-
- VALGRIND_MEMPOOL_ALLOC(context, ret, size);
-
- MemSetLoop(ret, 0, size);
-
- return ret;
-}
-
/*
* MemoryContextAllocExtended
* Allocate space within the specified context using the given flags.
diff --git a/src/include/c.h b/src/include/c.h
index 4b0f5138d83..26bf7ec16e7 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -1060,30 +1060,6 @@ extern void ExceptionalCondition(const char *conditionName,
} while (0)
-/*
- * MemSetTest/MemSetLoop are a variant version that allow all the tests in
- * MemSet to be done at compile time in cases where "val" and "len" are
- * constants *and* we know the "start" pointer must be word-aligned.
- * If MemSetTest succeeds, then it is okay to use MemSetLoop, otherwise use
- * MemSetAligned. Beware of multiple evaluations of the arguments when using
- * this approach.
- */
-#define MemSetTest(val, len) \
- ( ((len) & LONG_ALIGN_MASK) == 0 && \
- (len) <= MEMSET_LOOP_LIMIT && \
- MEMSET_LOOP_LIMIT != 0 && \
- (val) == 0 )
-
-#define MemSetLoop(start, val, len) \
- do \
- { \
- long * _start = (long *) (start); \
- long * _stop = (long *) ((char *) _start + (Size) (len)); \
- \
- while (_start < _stop) \
- *_start++ = 0; \
- } while (0)
-
/*
* Macros for range-checking float values before converting to integer.
* We must be careful here that the boundary values are expressed exactly
diff --git a/src/include/nodes/nodes.h b/src/include/nodes/nodes.h
index 4c32682e4ce..bc1dae98220 100644
--- a/src/include/nodes/nodes.h
+++ b/src/include/nodes/nodes.h
@@ -132,6 +132,7 @@ typedef struct Node
#define nodeTag(nodeptr) (((const Node*)(nodeptr))->type)
+
/*
* newNode -
* create a new node of the specified size and tag the node with the
@@ -139,39 +140,18 @@ typedef struct Node
*
* !WARNING!: Avoid using newNode directly. You should be using the
* macro makeNode. eg. to create a Query node, use makeNode(Query)
- *
- * Note: the size argument should always be a compile-time constant, so the
- * apparent risk of multiple evaluation doesn't matter in practice.
*/
-#ifdef __GNUC__
-
-/* With GCC, we can use a compound statement within an expression */
-#define newNode(size, tag) \
-({ Node *_result; \
- AssertMacro((size) >= sizeof(Node)); /* need the tag, at least */ \
- _result = (Node *) palloc0fast(size); \
- _result->type = (tag); \
- _result; \
-})
-#else
-
-/*
- * There is no way to dereference the palloc'ed pointer to assign the
- * tag, and also return the pointer itself, so we need a holder variable.
- * Fortunately, this macro isn't recursive so we just define
- * a global variable for this purpose.
- */
-extern PGDLLIMPORT Node *newNodeMacroHolder;
+static inline Node *
+newNode(size_t size, NodeTag tag)
+{
+ Node *result;
-#define newNode(size, tag) \
-( \
- AssertMacro((size) >= sizeof(Node)), /* need the tag, at least */ \
- newNodeMacroHolder = (Node *) palloc0fast(size), \
- newNodeMacroHolder->type = (tag), \
- newNodeMacroHolder \
-)
-#endif /* __GNUC__ */
+ Assert(size >= sizeof(Node)); /* need the tag, at least */
+ result = (Node *) palloc0(size);
+ result->type = tag;
+ return result;
+}
#define makeNode(_type_) ((_type_ *) newNode(sizeof(_type_),T_##_type_))
#define NodeSetTag(nodeptr,t) (((Node*)(nodeptr))->type = (t))
diff --git a/src/include/utils/palloc.h b/src/include/utils/palloc.h
index d1146c12351..cf98ddc0ec9 100644
--- a/src/include/utils/palloc.h
+++ b/src/include/utils/palloc.h
@@ -70,7 +70,6 @@ extern PGDLLIMPORT MemoryContext CurrentMemoryContext;
*/
extern void *MemoryContextAlloc(MemoryContext context, Size size);
extern void *MemoryContextAllocZero(MemoryContext context, Size size);
-extern void *MemoryContextAllocZeroAligned(MemoryContext context, Size size);
extern void *MemoryContextAllocExtended(MemoryContext context,
Size size, int flags);
extern void *MemoryContextAllocAligned(MemoryContext context,
@@ -109,19 +108,6 @@ extern void pfree(void *pointer);
#define repalloc_array(pointer, type, count) ((type *) repalloc(pointer, sizeof(type) * (count)))
#define repalloc0_array(pointer, type, oldcount, count) ((type *) repalloc0(pointer, sizeof(type) * (oldcount), sizeof(type) * (count)))
-/*
- * The result of palloc() is always word-aligned, so we can skip testing
- * alignment of the pointer when deciding which MemSet variant to use.
- * Note that this variant does not offer any advantage, and should not be
- * used, unless its "sz" argument is a compile-time constant; therefore, the
- * issue that it evaluates the argument multiple times isn't a problem in
- * practice.
- */
-#define palloc0fast(sz) \
- ( MemSetTest(0, sz) ? \
- MemoryContextAllocZeroAligned(CurrentMemoryContext, sz) : \
- MemoryContextAllocZero(CurrentMemoryContext, sz) )
-
/* Higher-limit allocators. */
extern void *MemoryContextAllocHuge(MemoryContext context, Size size);
extern pg_nodiscard void *repalloc_huge(void *pointer, Size size);
--
2.39.2
