[PATCH] D47917: [ARM] Lower llvm.ctlz.i32 to a libcall when clz is not available.

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

I'm not checking for optsize because I don't think it makes sense to inline even when optimizing for speed... although maybe that's not right.  The current code for Thumb1 is something like the following (which is essentially computing `popcount(nextpoweroftwo(x)-1)`).

  test:
          .fnstart
  @ %bb.0:
          lsrs    r1, r0, #1
          orrs    r1, r0
          lsrs    r0, r1, #2
          orrs    r0, r1
          lsrs    r1, r0, #4
          orrs    r1, r0
          lsrs    r0, r1, #8
          orrs    r0, r1
          lsrs    r1, r0, #16
          orrs    r1, r0
          mvns    r0, r1
          lsrs    r1, r0, #1
          ldr     r2, .LCPI0_0
          ands    r2, r1
          subs    r0, r0, r2
          ldr     r1, .LCPI0_1
          lsrs    r2, r0, #2
          ands    r0, r1
          ands    r2, r1
          adds    r0, r0, r2
          lsrs    r1, r0, #4
          adds    r0, r0, r1
          ldr     r1, .LCPI0_2
          ands    r1, r0
          ldr     r0, .LCPI0_3
          muls    r0, r1, r0
          lsrs    r0, r0, #24
          bx      lr
          .p2align        2
  @ %bb.1:
  .LCPI0_0:
          .long   1431655765              @ 0x55555555
  .LCPI0_1:
          .long   858993459               @ 0x33333333
  .LCPI0_2:
          .long   252645135               @ 0xf0f0f0f
  .LCPI0_3:
          .long   16843009                @ 0x1010101
  .Lfunc_end0:
          .size   test, .Lfunc_end0-test


Repository:
  rL LLVM

https://reviews.llvm.org/D47917