Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Aliasing

xania.org

Post reply on HN
Print view View original

31–36 of 36 posts

Re: Aliasing

#31

Post by gpvos »

gpderetta wrote: post #27
dataflow wrote: post #18

Earlier quoted context omitted.

I think this might not be a shortcoming of MSVC but rather a deliberate design decision. It seems likely that MSVC is failing to apply strict aliasing, but that it's deliberately avoiding it, probably for compatibility reasons with code that wasn't/isn't written to spec. And frankly it can be very onerous to write code that is 100% correct per the standard when dealing with e.g. memory-mapped files; I'm struggling to…

AFIK MSVC has never implemented TBAA by design.

TBAA = type-based alias analysis

Re: Aliasing

#32

Post by gpderetta »

aw1621107 wrote: post #30
newpavlov wrote: post #25

Earlier quoted context omitted.

LOL, nope. Those annotations must be part of the type system (e.g. `&mut T` in Rust) and must be checked by the compiler (the borrow checker). The language can provide escape hatches like `unsafe`, but they should be rarely used. Without it you get a fragile footgunny mess. Just look at the utter failure of `restrict`. It was so rarely used in C that it took several years of constant nagging from Rust developers to i…

Does make me wonder what restrict-related bugs will be (have been?) uncovered in GCC, if any. Or whether the GCC devs saw what LLVM went through and decided to try to address any issues preemptively.

gcc has had restrict for 25 years I think. I would hope most bugs have been squashed by now.

Re: Aliasing

#33

Post by aw1621107 »

gpderetta wrote: post #32
aw1621107 wrote: post #30

Earlier quoted context omitted.

Does make me wonder what restrict-related bugs will be (have been?) uncovered in GCC, if any. Or whether the GCC devs saw what LLVM went through and decided to try to address any issues preemptively.

gcc has had restrict for 25 years I think. I would hope most bugs have been squashed by now.

Possibly? LLVM had been around for a while as well but Rust still ended up running into aliasing-related optimizer bugs.

Now that I think about it some more, perhaps gfortran might be a differentiating factor? Not familiar enough with Fortran to guess as to how much it would exercise aliasing-related optimizations, though.

Re: Aliasing

#34

Post by newpavlov »

aw1621107 wrote: post #30
newpavlov wrote: post #25

Earlier quoted context omitted.

LOL, nope. Those annotations must be part of the type system (e.g. `&mut T` in Rust) and must be checked by the compiler (the borrow checker). The language can provide escape hatches like `unsafe`, but they should be rarely used. Without it you get a fragile footgunny mess. Just look at the utter failure of `restrict`. It was so rarely used in C that it took several years of constant nagging from Rust developers to i…

Does make me wonder what restrict-related bugs will be (have been?) uncovered in GCC, if any. Or whether the GCC devs saw what LLVM went through and decided to try to address any issues preemptively.

IIRC at least one of the `restrict` bugs found by Rust was reproduced on both LLVM and GCC.

Re: Aliasing

#35

Post by gpderetta »

aw1621107 wrote: post #33
gpderetta wrote: post #32

Earlier quoted context omitted.

gcc has had restrict for 25 years I think. I would hope most bugs have been squashed by now.

Possibly? LLVM had been around for a while as well but Rust still ended up running into aliasing-related optimizer bugs. Now that I think about it some more, perhaps gfortran might be a differentiating factor? Not familiar enough with Fortran to guess as to how much it would exercise aliasing-related optimizations, though.

I think Fortran function arguments are assumed not to alias. I'm not sure if it matches C restrict semantics though.

Re: Aliasing

#36

Post by aw1621107 »

gpderetta wrote: post #35
aw1621107 wrote: post #33

Earlier quoted context omitted.

Possibly? LLVM had been around for a while as well but Rust still ended up running into aliasing-related optimizer bugs. Now that I think about it some more, perhaps gfortran might be a differentiating factor? Not familiar enough with Fortran to guess as to how much it would exercise aliasing-related optimizations, though.

I think Fortran function arguments are assumed not to alias. I'm not sure if it matches C restrict semantics though.

Yeah, that's why I was wondering whether GCC might have shaken out its aliasing bugs. Sibling seems to recall otherwise, though.
Post reply on HN