7d790fcdfe
This uses the `cmov`, which was previously necessary for Spectre mitigation, to clamp the table index instead of zeroing it. By then placing the default target as the last entry in the table, we can use just one branch instruction in all cases. Since there isn't a bounds-check branch any more, this sequence no longer needs Spectre mitigation. And since we don't need to be careful about preserving flags, half the instructions can be removed from this pseudoinstruction and emitted as regular instructions instead. This is a net savings of three bytes in the encoding of x64's br_table pseudoinstruction. The generated code can sometimes be longer overall because the blocks are emitted in a slightly different order. My benchmark results show a very small effect on runtime performance with this change. The spidermonkey benchmark in Sightglass runs "1.01x faster" than main by instructions retired, but with no significant difference in CPU cycles. I think that means it rarely hit the default case in any br_table instructions it executed. The pulldown-cmark benchmark in Sightglass runs "1.01x faster" than main by CPU cycles, but main runs "1.00x faster" by instructions retired. I think that means this benchmark hit the default case a significant amount of the time, so it executes a few more instructions per br_table, but maybe the branches were predicted better.
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.