[llvm-opt-report] Incorrectly indicate vector length

[vfdff]

• According llvm-opt-report.html, the V is used to indicate vector length

  V: The loop is vectorized. The following numbers indicate the vector length and the interleave factor.

• demo.c :

__attribute__((noinline))
void demo (double * a, int N){
 for (int i = 0; i < N; i++)
   a[i] = 2.0 * a[i];
}

• issure reproduce: It show the vector length is 1 with the following instruction in file demo.opt.lst

step1: clang -Ofast -march=armv8.5-a+sve -mllvm -force-vector-interleave=1 -S demo.c -fsave-optimization-record
step2: llvm-opt-report demo.opt.yaml -o demo.opt.lst

[root@localhost 2.1]# cat demo.opt.lst
< demo.c
1      | __attribute__((noinline))
2      | void demo (double * a, int N){
3 V1,1 |  for (int i = 0; i < N; i++)
4      |    a[i] = 2.0 * a[i];
5      | }

• We can see the indicate vector length is 2 according the final output assemble file demo.s, which is different from the above report generated by llvm-opt-report.

.LBB0_7:                                // =>This Inner Loop Header: Depth=1
	.loc	1 4 17                          // demo.c:4:17
	ld1d	{ z0.d }, p0/z, [x0, x11, lsl 3]
	.loc	1 4 15 is_stmt 0                // demo.c:4:15
	fadd	z0.d, z0.d, z0.d
	.loc	1 4 9                           // demo.c:4:9
	st1d	{ z0.d }, p0, [x0, x11, lsl 3]
	.loc	1 3 26 is_stmt 1                // demo.c:3:26
	add	x11, x11, x10
	cmp	x9, x11
	b.ne	.LBB0_7

• BTW: opt-viewer.py can report correctly

  step1: clang -Ofast -march=armv8.5-a+sve -mllvm -force-vector-interleave=1 -S demo.c -fsave-optimization-record
  step2: python3 /path-to-clang/share/opt-viewer/opt-viewer.py --output-dir

[fhahn]

@vfdff do you know if this works as expected for fixed-width vector factors (i.e. when dropping +sve?) . the vector factor for the loop with +sve should be vscale x 2, it is possible that the tool doesn't parse scalable VFs properly

[vfdff]

Yes, it works fine for fixed-width vector factors. so it looks like your guess is correct.

• llvm-opt-report show the vector length is 2 with the following instruction in file demo.opt.lst

  step1: clang -Ofast -march=armv8.5-a -mllvm -force-vector-interleave=1 -S demo.c -fsave-optimization-record
  step2: llvm-opt-report demo.opt.yaml -o demo.opt.lst

< demo.c
1      | __attribute__((noinline))
2      | void demo (double * a, int N){
3 V2,1 |  for (int i = 0; i < N; i++)
4      |    a[i] = 2.0 * a[i];
5      | }

• We can also get the indicate vector length is 2 according the final output assemble file demo.s

.LBB0_7:                                // =>This Inner Loop Header: Depth=1
        .loc    1 4 17                          // demo.c:4:17
        ldr     q0, [x11]
        .loc    1 3 26                          // demo.c:3:26
        subs    x10, x10, #2
        .loc    1 4 15                          // demo.c:4:15
        fadd    v0.2d, v0.2d, v0.2d
        .loc    1 4 9 is_stmt 0                 // demo.c:4:9
        str     q0, [x11], #16
        .loc    1 3 26 is_stmt 1                // demo.c:3:26
        b.ne    .LBB0_7