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Re: [Qemu-devel] [PATCH 16/25] tcg/i386: Handle ctz and clz opcodes


From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH 16/25] tcg/i386: Handle ctz and clz opcodes
Date: Mon, 21 Nov 2016 11:37:28 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 11/18/2016 01:48 PM, Bastian Koppelmann wrote:
On 11/18/2016 12:03 AM, Richard Henderson wrote:
On 11/17/2016 11:09 PM, Bastian Koppelmann wrote:
On 11/17/2016 08:59 PM, Richard Henderson wrote:
On 11/17/2016 08:53 PM, Richard Henderson wrote:
On 11/17/2016 05:50 PM, Bastian Koppelmann wrote:
On 11/16/2016 08:25 PM, Richard Henderson wrote:
+
+    OP_32_64(clz):
+        if (const_args[2]) {
+            tcg_debug_assert(have_bmi1);
+            tcg_debug_assert(args[2] == (rexw ? 64 : 32));
+            tcg_out_modrm(s, OPC_LZCNT + rexw, args[0], args[1]);
+        } else {
+            /* ??? See above.  */
+            tcg_out_modrm(s, OPC_BSR + rexw, args[0], args[1]);

The Intel ISA manual states that it find the bit index of the most
significant bit, where the least significant bit is index 0. So for
the
input 0x2 this should return 1. However this is not the number of
leading zeros.

Oh, of course you're right.  I thought I was testing this, but while
alpha does
have this operation, it turns out it isn't used much.

Alternately, what I tested was on a haswell machine, which takes the
LZCNT path, which *does* produce the intended results.  Just the BSR
path doesn't.

Luckily my old laptop is a Core 2 Duo without LZCNT :)

Heh.  Well, I've given it another few tests with LZCNT hacked off, and
with i686 32-bit.  Here's an incremental update.  Wherein I also note
that lzcnt isn't in the same cpuid flag as tzcnt.  Double whoops.

My processor[1] seems to lie about the LZCNT cpuid flag. It says it has
LZCNT but executes it as BSR. According to [2] ABM flag is used to
indicate LZCNT support.

Yes, the gcc cpuid.h comment for the lzcnt bit, i.e. to which leaf it should apply, is wrong. I'll get that fixed in the next revision.


r~



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