This recognises the patterns of the form:
while (n & 1) { n >>= 1 }
Unfortunately there are currently two issues relating to this patch.
Firstly, simplify_using_initial_conditions does not recognise that
(n != 0) and ((n & 1) == 0) implies that ((n >> 1) != 0).
This preconditions arise following the loop copy-header pass, and the
assumptions returned by number_of_iterations_exit_assumptions then
prevent final value replacement from using the niter result.
I'm not sure what is the best way to fix this - one approach could be to
modify simplify_using_initial_conditions to handle this sort of case,
but it seems that it basically wants the information that ranger could
give anway, so would something like that be a better option?
The second issue arises in the vectoriser, which is able to determine
that the niter->assumptions are always true.
When building with -march=armv8.4-a+sve -S -O3, we get this codegen:
foo (unsigned int b) {
int c = 0;
if (b == 0)
return PREC;
while (!(b & (1 << (PREC - 1)))) {
b <<= 1;
c++;
}
return c;
}
foo:
.LFB0:
.cfi_startproc
cmp w0, 0
cbz w0, .L6
blt .L7
lsl w1, w0, 1
clz w2, w1
cmp w2, 14
bls .L8
mov x0, 0
cntw x3
add w1, w2, 1
index z1.s, #0, #1
whilelo p0.s, wzr, w1
.L4:
add x0, x0, x3
mov p1.b, p0.b
mov z0.d, z1.d
whilelo p0.s, w0, w1
incw z1.s
b.any .L4
add z0.s, z0.s, #1
lastb w0, p1, z0.s
ret
.p2align 2,,3
.L8:
mov w0, 0
b .L3
.p2align 2,,3
.L13:
lsl w1, w1, 1
.L3:
add w0, w0, 1
tbz w1, #31, .L13
ret
.p2align 2,,3
.L6:
mov w0, 32
ret
.p2align 2,,3
.L7:
mov w0, 0
ret
.cfi_endproc
In essence, the vectoriser uses the niter information to determine
exactly how many iterations of the loop it needs to run. It then uses
SVE whilelo instructions to run this number of iterations. The original
loop counter is also vectorised, despite only being used in the final
iteration, and then the final value of this counter is used as the
return value (which is the same as the number of iterations it computed
in the first place).
This vectorisation is obviously bad, and I think it exposes a latent
bug in the vectoriser, rather than being an issue caused by this
specific patch.
gcc/ChangeLog:
* tree-ssa-loop-niter.cc (number_of_iterations_cltz): New.
(number_of_iterations_bitcount): Add call to the above.
(number_of_iterations_exit_assumptions): Add EQ_EXPR case for
c[lt]z idiom recognition.
gcc/testsuite/ChangeLog:
* gcc.dg/tree-ssa/cltz-max.c: New test.
* gcc.dg/tree-ssa/clz-char.c: New test.
* gcc.dg/tree-ssa/clz-int.c: New test.
* gcc.dg/tree-ssa/clz-long-long.c: New test.
* gcc.dg/tree-ssa/clz-long.c: New test.
* gcc.dg/tree-ssa/ctz-char.c: New test.
* gcc.dg/tree-ssa/ctz-int.c: New test.
* gcc.dg/tree-ssa/ctz-long-long.c: New test.
* gcc.dg/tree-ssa/ctz-long.c: New test.
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See the file gcc/doc/gcc.texi (together with other files that it
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