Re: Simplify newNode()

On 15/12/2023 00:44, Tom Lane wrote:
>> 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.
> Not here ...

Hmm, according to godbolt, the change happened in GCC version 10.1. 
Starting with gcc 10.1, it is turned into a memset(). On clang, the same 
change happened in version 3.4.1.

I think we have consensus on patch v2. It's simpler and not less 
performant than what we have now, at least on modern compilers. Barring 
objections, I'll commit that.

I'm not planning to spend more time on this, but there might be some 
room for further optimization if someone is interested to do the 
micro-benchmarking. The obvious thing would be to persuade modern 
compilers to not switch to memset() in MemoryContextAllocZeroAligned 
(*), making the old macro logic work the same way it used to on old 
compilers.

Also, instead of palloc0, it might be better for newNode() to call 
palloc followed by memset. That would allow the compiler to partially 
optimize away the memset. Most callers fill at least some of the fields 
after calling makeNode(), so the compiler could generate code that 
clears only the uninitialized fields and padding bytes.

(*) or rather, a new function like MemoryContextAllocZeroAligned but 
without the 'context' argument. We want to keep the savings in the 
callers from eliminating the extra argument.

-- 
Heikki Linnakangas
Neon (https://neon.tech)