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[PATCH] x86: emit tzcnt unconditionally

tzcnt is encoded as "rep;bsf" and unlike lzcnt is a drop-in replacement
if we don't care about the flags (it has the same semantics for non-zero

Since bsf is usually slower, just emit tzcnt unconditionally.  However,
write it as rep;bsf unless -mbmi is in use, to cater for old assemblers.

Bootstrapped on a non-BMI x86_64-linux host, regtest in progress.
Ok for mainline?


2012-04-27  Paolo Bonzini  <>

        * config/i386/ (ctz<mode>2): Emit tzcnt (as rep;bsf)
        instead of bsf.

Index: i386/
--- i386/        (revisione 186904)
+++ i386/        (copia locale)
@@ -12082,14 +12082,15 @@ (define_insn "ctz<mode>2"
    (clobber (reg:CC FLAGS_REG))]
+  /* tzcnt expands to rep;bsf and we can use it even if !TARGET_BMI.  */
   if (TARGET_BMI)
     return "tzcnt{<imodesuffix>}\t{%1, %0|%0, %1}";
-    return "bsf{<imodesuffix>}\t{%1, %0|%0, %1}";
+    return "rep;bsf{<imodesuffix>}\t{%1, %0|%0, %1}";
   [(set_attr "type" "alu1")
    (set_attr "prefix_0f" "1")
-   (set (attr "prefix_rep") (symbol_ref "TARGET_BMI"))
+   (set_attr "prefix_rep" "1")
    (set_attr "mode" "<MODE>")])
 (define_expand "clz<mode>2"

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