fcddb9ca81
* x64: Refactor `Amode` computation in ISLE This commit replaces the previous computation of `Amode` with a different set of rules that are intended to achieve the same purpose but are structured differently. The motivation for this commit is going to become more relevant in the next commit where `lea` will be used for the `iadd` instruction, possibly, on x64. When doing so it caused a stack overflow in the test suite during the compilation phase of a wasm module, namely as part of the `amode_add` function. This function is recursively defined in terms of itself and recurses as deep as the deepest `iadd`-chain in a program. A particular test in our test suite has a 10k-long chain of `iadd` which ended up causing a stack overflow in debug mode. This stack overflow is caused because the `amode_add` helper in ISLE unconditionally peels all the `iadd` nodes away and looks at all of them, even if most end up in intermediate registers along the way. Given that structure I couldn't find a way to easily abort the recursion. The new `to_amode` helper is structured in a similar fashion but attempts to instead only recurse far enough to fold items into the final `Amode` instead of recursing through items which themselves don't end up in the `Amode`. Put another way previously the `amode_add` helper might emit `x64_add` instructions, but it no longer does that. This goal of this commit is to preserve all the original `Amode` optimizations, however. For some parts, though, it relies more on egraph optimizations to run since if an `iadd` is 10k deep it doesn't try to find a constant buried 9k levels inside there to fold into the `Amode`. The hope, though, is that with egraphs having run already it's shuffled constants to the right most of the time and already folded any possible together. * x64: Add `lea`-based lowering for `iadd` This commit adds a rule for the lowering of `iadd` to use `lea` for 32 and 64-bit addition. The theoretical benefit of `lea` over the `add` instruction is that the `lea` variant can emulate a 3-operand instruction which doesn't destructively modify on of its operands. Additionally the `lea` operation can fold in other components such as constant additions and shifts. In practice, however, if `lea` is unconditionally used instead of `iadd` it ends up losing 10% performance on a local `meshoptimizer` benchmark. My best guess as to what's going on here is that my CPU's dedicated units for address computation are all overloaded while the ALUs are basically idle in a memory-intensive loop. Previously when the ALU was used for `add` and the address units for stores/loads it in theory pipelined things better (most of this is me shooting in the dark). To prevent the performance loss here I've updated the lowering of `iadd` to conditionally sometimes use `lea` and sometimes use `add` depending on how "complicated" the `Amode` is. Simple ones like `a + b` or `a + $imm` continue to use `add` (and its subsequent hypothetical extra `mov` necessary into the result). More complicated ones like `a + b + $imm` or `a + b << c + $imm` use `lea` as it can remove the need for extra instructions. Locally at least this fixes the performance loss relative to unconditionally using `lea`. One note is that this adds an `OperandSize` argument to the `MInst::LoadEffectiveAddress` variant to add an encoding for 32-bit `lea` in addition to the preexisting 64-bit encoding. * Conditionally use `lea` based on regalloc
filetests
Filetests is a crate that contains multiple test suites for testing
various parts of cranelift. Each folder under cranelift/filetests/filetests is a different
test suite that tests different parts.
Adding a runtest
One of the available testsuites is the "runtest" testsuite. Its goal is to compile some piece of clif code, run it and ensure that what comes out is what we expect.
To build a run test you can add the following to a file:
test interpret
test run
target x86_64
target aarch64
target s390x
function %band_f32(f32, f32) -> f32 {
block0(v0: f32, v1: f32):
v2 = band v0, v1
return v2
}
; run: %band_f32(0x0.5, 0x1.0) == 0x1.5
Since this is a run test for band we can put it in: runtests/band.clif.
Once we have the file in the test suite we can run it by invoking: cargo run -- test filetests/filetests/runtests/band.clif from the cranelift directory.
The first lines tell clif-util what kind of tests we want to run on this file.
test interpret invokes the interpreter and checks if the conditions in the ; run comments pass. test run does the same, but compiles the file and runs it as a native binary.
For more information about testing see testing.md.