spatel added a comment.
In D87188#2281957 <https://reviews.llvm.org/D87188#2281957>, @sanwou01 wrote:
> I know this has already been reverted but just FYI that I've bisected a ~2%
> regression in SPEC2017 x264_r on AArch64 to this commit. Presumably this is
> due to the extra unrolling / cost modelling issue already mentioned?
That would be my guess. I'm not familiar with the unroller or inliner, but I
don't even see them using the cost model at 1st glance. Are they purely
counting IR instructions?
If I missed the cost model calls, the cost model is still not correct for
`code-size`:
$ cat cost.ll
declare i32 @llvm.abs.i32(i32, i1)
define i32 @abs(i32 %x, i32 %y) {
%I32 = call i32 @llvm.abs.i32(i32 %x, i1 false)
%negx = sub i32 0, %x
%cmp = icmp sgt i32 %x, -1
%sel = select i1 %cmp, i32 %x, i32 %negx
ret i32 %I32
}
$ opt -cost-model -cost-kind=code-size -analyze -mtriple=aarch64-- cost.ll
Printing analysis 'Cost Model Analysis' for function 'abs':
Cost Model: Found an estimated cost of 1 for instruction: %I32 = call i32
@llvm.abs.i32(i32 %x, i1 false)
Cost Model: Found an estimated cost of 1 for instruction: %negx = sub i32
0, %x
Cost Model: Found an estimated cost of 1 for instruction: %cmp = icmp sgt
i32 %x, -1
Cost Model: Found an estimated cost of 1 for instruction: %sel = select i1
%cmp, i32 %x, i32 %negx
So we're assuming the size cost is either 1 or 3 depending on how we encoded
abs() in IR, but neither is correct for AArch64 IIUC:
$ llc -o - -mtriple=aarch64-- cost.ll
cmp w0, #0 // =0
cneg w0, w0, mi
ret
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D87188/new/
https://reviews.llvm.org/D87188
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
