Subject: Re: [PATCH] powerpc: emulate power5 popcntb instruction

From: Willschm <[email-masked]>
Date: Mon, 28 Aug 2006 13:03:35 -0500

On Thu, 2006-24-08 at 16:36 +1000, Paul Mackerras wrote:
> Will Schmidt writes:
> 
> I just did a patch to fix the existing masks.  Could you do a new
> version of this patch that doesn't include the unrelated mask fixes
> please?  Also it would be really nice if you could figure out a way to
> avoid doing the unnecessary 64-bit logical operations on 32-bit
> machines - i.e. using an unsigned long for tmp, but then the constants
> become problematic.  Maybe you need something like
> 
> #define LCONST(x)	((unsigned long)(x##ULL))

Ok, how about this..

In an attempt to make it easier for a power5 optimized app to run on a 
power4 or a 970 or random earlier machine, this provides emulation of
the popcntb instruction.

Signed-off-by: Will Schmidt <[email-masked]>

---
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 2105767..f85a3af 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -588,6 +588,9 @@ #define INST_LSWX		0x7c00042a
 #define INST_STSWI		0x7c0005aa
 #define INST_STSWX		0x7c00052a
 
+#define INST_POPCNTB		0x7c0000f4
+#define INST_POPCNTB_MASK	0xfc0007fe
+
 static int emulate_string_inst(struct pt_regs *regs, u32 instword)
 {
 	u8 rT = (instword >> 21) & 0x1f;
@@ -656,6 +659,25 @@ static int emulate_string_inst(struct pt
 	return 0;
 }
 
+#define LCONST(x)	((unsigned long)(x##ULL))
+
+static int emulate_popcntb_inst(struct pt_regs *regs, u32 instword)
+{
+	u32 ra,rs;
+	u64 tmp;
+
+	ra = (instword >> 16) & 0x1f;
+	rs = (instword >> 21) & 0x1f;
+
+	tmp = regs->gpr[rs];
+	tmp = tmp - ((tmp >> 1) & LCONST(0x5555555555555555));
+	tmp = (tmp & LCONST(0x3333333333333333)) + ((tmp >> 2) & LCONST(0x3333333333333333));
+	tmp = (tmp + (tmp >> 4)) & LCONST(0x0f0f0f0f0f0f0f0f);
+	regs->gpr[ra] = tmp;
+
+	return 0;
+}
+
 static int emulate_instruction(struct pt_regs *regs)
 {
 	u32 instword;
@@ -693,6 +715,11 @@ static int emulate_instruction(struct pt
 	if ((instword & INST_STRING_GEN_MASK) == INST_STRING)
 		return emulate_string_inst(regs, instword);
 
+	/* Emulate the popcntb (Population Count Bytes) instruction. */
+	if ((instword & INST_POPCNTB_MASK) == INST_POPCNTB) {
+		return emulate_popcntb_inst(regs, instword);
+	}
+
 	return -EINVAL;
 }