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This commit improves code generation for WebAssembly targets when
translating floating to integer casts. This improvement is only relevant
when the `nontrapping-fptoint` feature is not enabled, but the feature
is not enabled by default right now. Additionally this improvement only
affects safe casts since unchecked casts were improved in #74659.
Some more background for this issue is present on #73591, but the
general gist of the issue is that in LLVM the `fptosi` and `fptoui`
instructions are defined to return an `undef` value if they execute on
out-of-bounds values; they notably do not trap. To implement these
instructions for WebAssembly the LLVM backend must therefore generate
quite a few instructions before executing `i32.trunc_f32_s` (for
example) because this WebAssembly instruction traps on out-of-bounds
values. This codegen into wasm instructions happens very late in the
code generator, so what ends up happening is that rustc inserts its own
codegen to implement Rust's saturating semantics, and then LLVM also
inserts its own codegen to make sure that the `fptosi` instruction
doesn't trap. Overall this means that a function like this:
#[no_mangle]
pub unsafe extern "C" fn cast(x: f64) -> u32 {
x as u32
}
will generate this WebAssembly today:
(func $cast (type 0) (param f64) (result i32)
(local i32 i32)
local.get 0
f64.const 0x1.fffffffep+31 (;=4.29497e+09;)
f64.gt
local.set 1
block ;; label = @1
block ;; label = @2
local.get 0
f64.const 0x0p+0 (;=0;)
local.get 0
f64.const 0x0p+0 (;=0;)
f64.gt
select
local.tee 0
f64.const 0x1p+32 (;=4.29497e+09;)
f64.lt
local.get 0
f64.const 0x0p+0 (;=0;)
f64.ge
i32.and
i32.eqz
br_if 0 (;@2;)
local.get 0
i32.trunc_f64_u
local.set 2
br 1 (;@1;)
end
i32.const 0
local.set 2
end
i32.const -1
local.get 2
local.get 1
select)
This PR improves the situation by updating the code generation for
float-to-int conversions in rustc, specifically only for WebAssembly
targets and only for some situations (float-to-u8 still has not great
codegen). The fix here is to use basic blocks and control flow to avoid
speculatively executing `fptosi`, and instead LLVM's raw intrinsic for
the WebAssembly instruction is used instead. This effectively extends
the support added in #74659 to checked casts. After this commit the
codegen for the above Rust function looks like:
(func $cast (type 0) (param f64) (result i32)
(local i32)
block ;; label = @1
local.get 0
f64.const 0x0p+0 (;=0;)
f64.ge
local.tee 1
i32.const 1
i32.xor
br_if 0 (;@1;)
local.get 0
f64.const 0x1.fffffffep+31 (;=4.29497e+09;)
f64.le
i32.eqz
br_if 0 (;@1;)
local.get 0
i32.trunc_f64_u
return
end
i32.const -1
i32.const 0
local.get 1
select)
For reference, in Rust 1.44, which did not have saturating
float-to-integer casts, the codegen LLVM would emit is:
(func $cast (type 0) (param f64) (result i32)
block ;; label = @1
local.get 0
f64.const 0x1p+32 (;=4.29497e+09;)
f64.lt
local.get 0
f64.const 0x0p+0 (;=0;)
f64.ge
i32.and
i32.eqz
br_if 0 (;@1;)
local.get 0
i32.trunc_f64_u
return
end
i32.const 0)
So we're relatively close to the original codegen, although it's
slightly different because the semantics of the function changed where
we're emulating the `i32.trunc_sat_f32_s` instruction rather than always
replacing out-of-bounds values with zero.
There is still work that could be done to improve casts such as `f32` to
`u8`. That form of cast still uses the `fptosi` instruction which
generates lots of branch-y code. This seems less important to tackle now
though. In the meantime this should take care of most use cases of
floating-point conversion and as a result I'm going to speculate that
this...
Closes #73591
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Use WASM's saturating casts if they are available
WebAssembly supports saturating floating point to integer casts behind a target feature. The feature is already available on many browsers. Beginning with 1.45 Rust will start defining the behavior of floating point to integer casts to be saturating as well. For this Rust constructs additional checks on top of the `fptoui` / `fptosi` instructions it emits. Here we introduce the possibility for the codegen backend to construct saturating casts itself and only fall back to constructing the checks ourselves if that is not possible.
Resolves part of #73591
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WebAssembly supports saturating floating point to integer casts behind a
target feature. The feature is already available on many browsers.
Beginning with 1.45 Rust will start defining the behavior of floating
point to integer casts to be saturating as well. For this Rust
constructs additional checks on top of the `fptoui` / `fptosi`
instructions it emits. Here we introduce the possibility for the codegen
backend to construct saturating casts itself and only fall back to
constructing the checks ourselves if that is not possible.
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added regions with counter expressions and counters.
Added codegen_llvm/coverageinfo mod for upcoming coverage map
Move coverage region collection to CodegenCx finalization
Moved from `query coverageinfo` (renamed from `query coverage_data`),
as discussed in the PR at:
https://github.com/rust-lang/rust/pull/73684#issuecomment-649882503
Address merge conflict in MIR instrument_coverage test
The MIR test output format changed for int types.
moved debug messages out of block.rs
This makes the block.rs calls to add coverage mapping data to the
CodegenCx much more concise and readable.
move coverage intrinsic handling into llvm impl
I realized that having half of the coverage intrinsic handling in
`rustc_codegen_ssa` and half in `rustc_codegen_llvm` meant that any
non-llvm backend would be bound to the same decisions about how the
coverage-related MIR terminators should be handled.
To fix this, I moved the non-codegen portion of coverage intrinsic
handling into its own trait, and implemented it in `rustc_codegen_llvm`
alongside `codegen_intrinsic_call`.
I also added the (required?) stubs for the new intrinsics to
`IntrepretCx::emulate_intrinsic()`, to ensure calls to this function do
not fail if called with these new but known intrinsics.
address PR Feedback on 28 June 2020 2:48pm PDT
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This initial version only injects counters at the top of each function.
Rust Coverage will require injecting additional counters at each
conditional code branch.
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`count_insn` is no longer called for inline asm, because it is private to builder.rs
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BuilderMethods::checked_binop
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