On Tue, Jul 25, 2017 at 8:24 PM, David Laight [off-list ref] wrote:

From: Linuxppc-dev [mailto:linuxppc-dev-bounces+david.laight=aculab.com@lists.ozlabs.org] On Behalf Of

quoted

Matt Brown
Sent: 25 July 2017 04:33
To: linuxppc-dev@lists.ozlabs.org
Subject: [PATCH v3 2/5] powerpc/lib/sstep: Add popcnt instruction emulation

This adds emulations for the popcntb, popcntw, and popcntd instructions.
Tested for correctness against the popcnt{b,w,d} instructions on ppc64le.

Signed-off-by: Matt Brown <redacted>
---
v3:
      - optimised using the Giles-Miller method of side-ways addition
v2:
      - fixed opcodes
      - fixed typecasting
      - fixed bitshifting error for both 32 and 64bit arch
---
 arch/powerpc/lib/sstep.c | 40 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
index 87d277f..c1f9cdb 100644
--- a/arch/powerpc/lib/sstep.c
+++ b/arch/powerpc/lib/sstep.c

@@ -612,6 +612,32 @@ static nokprobe_inline void do_cmpb(struct pt_regs *regs, unsigned long v1,
      regs->gpr[rd] = out_val;
 }

+/*
+ * The size parameter is used to adjust the equivalent popcnt instruction.
+ * popcntb = 8, popcntw = 32, popcntd = 64
+ */
+static nokprobe_inline void do_popcnt(struct pt_regs *regs, unsigned long v1,
+                             int size, int ra)
+{
+     unsigned long long out = v1;
+
+     out = (0x5555555555555555 & out) + (0x5555555555555555 & (out >> 1));
+     out = (0x3333333333333333 & out) + (0x3333333333333333 & (out >> 2));
+     out = (0x0f0f0f0f0f0f0f0f & out) + (0x0f0f0f0f0f0f0f0f & (out >> 4));
+     if (size == 8) {        /* popcntb */
+             regs->gpr[ra] = out;

I'm pretty sure you need to mask the result with 7.

Absolutely! Good catch!

Balbir Singh.