optimize swaps #6710
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optimize swaps #6710
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This isn't needed semantically, and it's the wrong case to optimize for.
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I'm just curious, but is this measurably faster in the sense that LLVM has been actually seen to generate faster code? I realize that in theory it's an improvement, but I'm curious if there's something currently which actually improves from this. |
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Yeah, it looked like an OK change to me but I really should have asked for benchmarks. |
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@alexcrichton: dropping the equality check is definitely an improvement I can see in the IR for I doubt it would show up in benchmarks for most code, but it's a measurable little improvement for the tree balancing. For very large objects, it would probably be helpful to expose alignment information to LLVM too, but I'll deal with that separately once this lands (we are always passing 1 as the alignment to |
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Passing higher alignment values gives the optimization passes more freedom since it can copy in larger chunks. This change results in rustc outputting the same post-optimization IR as clang for swaps and most copies excluding the lack of information about padding.
Code snippet:
```rust
#[inline(never)]
fn swap<T>(x: &mut T, y: &mut T) {
util::swap(x, y);
}
```
Original IR (for `int`):
```llvm
define internal fastcc void @_ZN9swap_283417_a71830ca3ed2d65d3_00E(i64*, i64*) #1 {
static_allocas:
%2 = icmp eq i64* %0, %1
br i1 %2, label %_ZN4util9swap_283717_a71830ca3ed2d65d3_00E.exit, label %3
; <label>:3 ; preds = %static_allocas
%4 = load i64* %0, align 1
%5 = load i64* %1, align 1
store i64 %5, i64* %0, align 1
store i64 %4, i64* %1, align 1
br label %_ZN4util9swap_283717_a71830ca3ed2d65d3_00E.exit
_ZN4util9swap_283717_a71830ca3ed2d65d3_00E.exit: ; preds = %3, %static_allocas
ret void
}
```
After #6710:
```llvm
define internal fastcc void @_ZN9swap_283017_a71830ca3ed2d65d3_00E(i64* nocapture, i64* nocapture) #1 {
static_allocas:
%2 = load i64* %0, align 1
%3 = load i64* %1, align 1
store i64 %3, i64* %0, align 1
store i64 %2, i64* %1, align 1
ret void
}
```
After this change:
```llvm
define internal fastcc void @_ZN9swap_283017_a71830ca3ed2d65d3_00E(i64* nocapture, i64* nocapture) #1 {
static_allocas:
%2 = load i64* %0, align 8
%3 = load i64* %1, align 8
store i64 %3, i64* %0, align 8
store i64 %2, i64* %1, align 8
ret void
}
```
Another example:
```rust
#[inline(never)]
fn set<T>(x: &mut T, y: T) {
*x = y;
}
```
Before, with `(int, int)` (align 1):
```llvm
define internal fastcc void @_ZN8set_282517_8fa972e3f9e451983_00E({ i64, i64 }* nocapture, { i64, i64 }* nocapture) #1 {
static_allocas:
%2 = bitcast { i64, i64 }* %1 to i8*
%3 = bitcast { i64, i64 }* %0 to i8*
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 16, i32 1, i1 false)
ret void
}
```
After, with `(int, int)` (align 8):
```llvm
define internal fastcc void @_ZN8set_282617_8fa972e3f9e451983_00E({ i64, i64 }* nocapture, { i64, i64 }* nocapture) #1 {
static_allocas:
%2 = bitcast { i64, i64 }* %1 to i8*
%3 = bitcast { i64, i64 }* %0 to i8*
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 16, i32 8, i1 false)
ret void
}
```
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