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ARM support: provide compiler a better popcount function

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Just the 32-bit one for now. The default uses lookup tables and is
ungainly and bloated.

Change-Id: I4a2eb31defb1f4d6f6853b65fe6dacc380d6ffc0
This commit is contained in:
Michael Sevakis 2017-09-07 15:41:52 -04:00
parent 28591f2e92
commit c6d5cd74a8
1 changed files with 31 additions and 0 deletions
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31
lib/arm_support/support-arm.S
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@ -701,3 +701,34 @@ __aeabi_idivmod:
.byte 0x92, 0x90, 0x8f, 0x8e, 0x8d, 0x8c, 0x8a, 0x89
.byte 0x88, 0x87, 0x86, 0x85, 0x84, 0x83, 0x82, 0x81
#endif
/*
* int __popcountsi2(unsigned int x)
* int __popcountdi2(unsigned long x)
*/
.section .text.__popcountsi2, "ax", %progbits
.global __popcountsi2
.type __popcountsi2, %function
.global __popcountdi2
.type __popcountdi2, %function
.set __popcountdi2, __popcountsi2
__popcountsi2:
mov r1, #0x33 @ r1 = 0x33333333
orr r1, r1, r1, lsl #8 @ ...
orr r1, r1, r1, lsl #16 @ ...
eor r2, r1, r1, lsl #1 @ r2 = 0x55555555
and r2, r2, r0, lsr #1 @ r2 = (x >> 1) & 0x55555555
sub r0, r0, r2 @ x = x - ((x >> 1) & 0x55555555)
and r2, r1, r0 @ r2 = x & 0x33333333
and r1, r1, r0, lsr #2 @ r1 = (x >> 2) & 0x33333333
add r0, r2, r1 @ x = (x & 0x33333333) + ((x >> 2) & 0x33333333)
mov r1, #0x0f @ r1 = 0x0f0f0f0f
orr r1, r1, r1, lsl #8 @ ...
orr r1, r1, r1, lsl #16 @ ...
add r0, r0, lsr #4 @ x = x + (x >> 4)
and r0, r0, r1 @ x = (x + (x >> 4)) & 0x0f0f0f0f
add r0, r0, lsr #16 @ x = x + (x >> 16)
add r0, r0, lsr #8 @ x = x + (x >> 8)
and r0, r0, #0x3f @ x &= 0x3f
bx lr @ return x
.size __popcountsi2, .-__popcountsi2