Re: [PATCH 3/6] powerpc sstep: Add cnttzw, cnttzd instruction emulation

On Tue, Sep 04, 2018 at 04:12:07PM -0500, Segher Boessenkool wrote:

On Mon, Sep 03, 2018 at 08:49:35PM +0530, Sandipan Das wrote:

quoted

+		case 538:	/* cnttzw */
+			if (!cpu_has_feature(CPU_FTR_ARCH_300))
+				return -1;
+			val = (unsigned int) regs->gpr[rd];
+			op->val = ( val ? __builtin_ctz(val) : 32 );
+			goto logical_done;
+#ifdef __powerpc64__
+		case 570:	/* cnttzd */
+			if (!cpu_has_feature(CPU_FTR_ARCH_300))
+				return -1;
+			val = regs->gpr[rd];
+			op->val = ( val ? __builtin_ctzl(val) : 64 );
+			goto logical_done;

__builtin_ctz(val) is undefined for val == 0.

Which would be why he only calls it when val != 0, presumably, and
uses 64 when val == 0.  Apart from idiosyncratic whitespace his code
looks correct to me.

Are you saying there is a bug in his code, or that his patch
description is incomplete, or what?

Paul.