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Redirect __rust_dealloc to sdallocx #122329

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Redirect __rust_dealloc to sdallocx #122329

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9 changes: 9 additions & 0 deletions compiler/rustc/src/main.rs
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Expand Up @@ -34,6 +34,15 @@
// https://github.com/rust-lang/rust/commit/b90cfc887c31c3e7a9e6d462e2464db1fe506175#diff-43914724af6e464c1da2171e4a9b6c7e607d5bc1203fa95c0ab85be4122605ef
// for an example of how to do so.

/// This redirects `__rust_dealloc` to jemalloc so it can make use of the size of the allocation.
#[cfg(feature = "jemalloc-sys")]
#[no_mangle]
pub unsafe extern "C" fn __rust_dealloc(ptr: *mut u8, size: usize, _align: usize) {
unsafe { jemalloc_sys::sdallocx(ptr.cast(), size, 0) }
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@Zoxc I think you closed this PR to try using a #[global_allocator] here with the statically linked std, but just as a heads up:

  • I had quickly tried using jemalloc as a global allocator in the rustc driver and it segfaulted while building std
  • otherwise, don't we also need to take alignment into account in the __rust_dealloc code above in the general case, so that it matches what System.alloc does for big alignments and allocations w/ a smaller size than the alignment? That being said, in the context of rustc it shouldn't matter, these values should be coming from types that wouldn't need non-zero jemalloc flags. I did a perf run with both 0, and flags computed by checking size/alignment, and it seemed neither made any improvement over the master branch. Weird, after seeing wins in this PR.

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  • I had quickly tried using jemalloc as a global allocator in the rustc driver and it segfaulted while building std

Do you have a branch for that?

  • otherwise, don't we also need to take alignment into account in the __rust_dealloc code above

My understanding is that the alignment argument is just a performance hint for sdallocx.

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I don't think that it's just a performance hint, because it modifies the actual size (jemalloc/jemalloc-cmake@4cfe551#diff-a4cb09e38cfec8141b07c291f731a8e01a17412568a852884fd921e8e521766bR1850 - this is some old code, the new one is more complicated, but does the same thing). Sadly, it also seems like sdallocx takes a slow path when flags != 0, although that might not happen that often.

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Do you have a branch for that?

yes, some wip work here lqd@6819e3c

rustc-main symbols override are still there as their presence/absence didn't seem to impact the segfault, but maybe it does and llvm would need to be setup differently.

#[used]
static _USED: unsafe extern "C" fn(*mut u8, usize, usize) = __rust_dealloc;
}

#[unix_sigpipe = "sig_dfl"]
fn main() {
// See the comment at the top of this file for an explanation of this.
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