Let's -disable-post-ra for x86

Project Infrastructure LLVM Dev List Archives
#1

We are currently running the post-ra scheduler for x86 with -O3. It is not really clear that it is worth the compile time.

X86 scheduling is a mystery that post-ra has not solved.

I ran the nightly test suite on x86_64 with LTO enabled and -O3 with and without -disable-post-ra. Code generation is 10% faster overall with -disable-post-ra, and the runtime impact varies from a 17% speedup to a 17 percent slowdown.

Below are the tests that had a more than 3% change in runtime.

I think we should disable post-ra scheduling for x86.

disable-post-ra.pdf (28 KB)

#2

Hi Jakob,

It's not completely clear from your post, I think your chart is just
for x86_64? What does it look like for x86_32?

- Daniel

#3

You're right, those were 64-bit numbers.

32-bit coming up.

#4

Here are the 32-bit numbers. I have omitted the compile times, since that is pretty clearly a universal win.

The table has runtime of llc-built tests with -O3 and LTO enabled, TARGET_FLAGS='-arch i386'.

m32-disable-post-ra.pdf (29.7 KB)