This detects (a subset of) the cases when `transmute::<T, U>(x)` can be
lowered to a direct `bitcast T x to U` in LLVM. This assists with
efficiently handling a SIMD vector as multiple different types,
e.g. swapping bytes/words/double words around inside some larger vector
type.
C compilers like GCC and Clang handle integer vector types as `__m128i`
for all widths, and implicitly insert bitcasts as required. This patch
allows Rust to express this, even if it takes a bit of `unsafe`, whereas
previously it was impossible to do at all without inline assembly.
Example:
pub fn reverse_u32s(u: u64x2) -> u64x2 {
unsafe {
let tmp = mem::transmute::<_, u32x4>(u);
let swapped = u32x4(tmp.3, tmp.2, tmp.1, tmp.0);
mem::transmute::<_, u64x2>(swapped)
}
}
Compiling with `--opt-level=3` gives:
Before
define <2 x i64> @_ZN12reverse_u32s20hbdb206aba18a03d8tbaE(<2 x i64>) unnamed_addr #0 {
entry-block:
%1 = bitcast <2 x i64> %0 to i128
%u.0.extract.trunc = trunc i128 %1 to i32
%u.4.extract.shift = lshr i128 %1, 32
%u.4.extract.trunc = trunc i128 %u.4.extract.shift to i32
%u.8.extract.shift = lshr i128 %1, 64
%u.8.extract.trunc = trunc i128 %u.8.extract.shift to i32
%u.12.extract.shift = lshr i128 %1, 96
%u.12.extract.trunc = trunc i128 %u.12.extract.shift to i32
%2 = insertelement <4 x i32> undef, i32 %u.12.extract.trunc, i64 0
%3 = insertelement <4 x i32> %2, i32 %u.8.extract.trunc, i64 1
%4 = insertelement <4 x i32> %3, i32 %u.4.extract.trunc, i64 2
%5 = insertelement <4 x i32> %4, i32 %u.0.extract.trunc, i64 3
%6 = bitcast <4 x i32> %5 to <2 x i64>
ret <2 x i64> %6
}
_ZN12reverse_u32s20hbdb206aba18a03d8tbaE:
.cfi_startproc
movd %xmm0, %rax
punpckhqdq %xmm0, %xmm0
movd %xmm0, %rcx
movq %rcx, %rdx
shrq $32, %rdx
movq %rax, %rsi
shrq $32, %rsi
movd %eax, %xmm0
movd %ecx, %xmm1
punpckldq %xmm0, %xmm1
movd %esi, %xmm2
movd %edx, %xmm0
punpckldq %xmm2, %xmm0
punpckldq %xmm1, %xmm0
retq
After
define <2 x i64> @_ZN12reverse_u32s20hbdb206aba18a03d8tbaE(<2 x i64>) unnamed_addr #0 {
entry-block:
%1 = bitcast <2 x i64> %0 to <4 x i32>
%2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
%3 = bitcast <4 x i32> %2 to <2 x i64>
ret <2 x i64> %3
}
_ZN12reverse_u32s20hbdb206aba18a03d8tbaE:
.cfi_startproc
pshufd $27, %xmm0, %xmm0
retq
Part of enforcing capacity-related conventions, for #18424, the collections reform.
Implements `fn shrink_to_fit` for HashMap.
The `reserve` method now takes as an argument the *extra* space to reserve.
As an example of what this changes, the following code:
let x: [int ..4];
Currently spits out ‘expected `]`, found `..`’. However, a comma would also be
valid there, as would a number of other tokens. This change adjusts the parser
to produce more accurate errors, so that that example now produces ‘expected one
of `(`, `+`, `,`, `::`, or `]`, found `..`’.
This closes#19168. It's possible that if the downloading of `rustup.sh`
is interrupted, bad things could happen, such as running a naked
"rm -rf /" instead of "rm -rf /path/to/tmpdir". This wraps rustup.sh's
functionality in a function that gets called at the last time that should
protect us from these truncation errors.
Treat builtin bounds like all other kinds of trait matches. Introduce a simple hashset in the fulfillment context to catch cases where we register the exact same obligation twice. This helps prevent duplicate error reports but also handles the recursive obligations created by builtin bounds.
r? @pcwalton
cc @FlaPer87
Otherwise the generated documentation is 30% larger. The sidebar
renders an entry for each item to all items, so large modules have
O(n^2) items rendered in the sidebars. Not a correct solution, but
at least it works.
They are just (unsafe) functions and static items to most users
and even compilers! The metadata doesn't distinguish them, so Rustdoc
ended up producing broken links (generated `ffi.*.html`, links to
`fn.*.html`). It would be best to avoid this pitfall at all.
One negative side-effect of this change is that there might be quite a bit of copying strings out of the codemap, i.e. one copy for every block that gets translated, just for taking a look at the last character of the block. If this turns out to cause a performance problem then `CodeMap::span_to_snippet()` could be changed return `Option<&str>` instead of `Option<String>`.
Fixes#18791
