[commit: ghc] master: Revert "Use __builtin_clz() to implement log_2()" (546f24e)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/546f24e4f8a7c086b1e5afcdda624176610cbcf8/ghc

>---------------------------------------------------------------

commit 546f24e4f8a7c086b1e5afcdda624176610cbcf8
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu Apr 28 15:20:43 2016 +0100

    Revert "Use __builtin_clz() to implement log_2()"
    
    This reverts commit 24864ba5587c1a0447beabae90529e8bb4fa117a.


>---------------------------------------------------------------

546f24e4f8a7c086b1e5afcdda624176610cbcf8
 rts/sm/BlockAlloc.c | 32 +++++++++++---------------------
 1 file changed, 11 insertions(+), 21 deletions(-)

diff --git a/rts/sm/BlockAlloc.c b/rts/sm/BlockAlloc.c
index 1c83de9..a633726 100644
--- a/rts/sm/BlockAlloc.c
+++ b/rts/sm/BlockAlloc.c
@@ -199,41 +199,31 @@ initGroup(bdescr *head)
   }
 }
 
-// log base 2 (floor), needs to support up to 2^MAX_FREE_LIST
+// There are quicker non-loopy ways to do log_2, but we expect n to be
+// usually small, and MAX_FREE_LIST is also small, so the loop version
+// might well be the best choice here.
 STATIC_INLINE nat
-log_2(W_ n)
+log_2_ceil(W_ n)
 {
-#if defined(__GNUC__)
-    return __builtin_clzl(n) ^ (sizeof(StgWord)*8 - 1);
-    // generates good code on x86.  __builtin_clz() compiles to bsr+xor, but
-    // we want just bsr, so the xor here cancels out gcc's xor.
-#else
     W_ i, x;
-    x = n;
+    x = 1;
     for (i=0; i < MAX_FREE_LIST; i++) {
-        x = x >> 1;
-        if (x == 0) return i;
+        if (x >= n) return i;
+        x = x << 1;
     }
     return MAX_FREE_LIST;
-#endif
 }
 
-// log base 2 (ceiling), needs to support up to 2^MAX_FREE_LIST
 STATIC_INLINE nat
-log_2_ceil(W_ n)
+log_2(W_ n)
 {
-#if defined(__GNUC__)
-    nat r = log_2(n);
-    return (n & (n-1)) ? r+1 : r;
-#else
     W_ i, x;
-    x = 1;
+    x = n;
     for (i=0; i < MAX_FREE_LIST; i++) {
-        if (x >= n) return i;
-        x = x << 1;
+        x = x >> 1;
+        if (x == 0) return i;
     }
     return MAX_FREE_LIST;
-#endif
 }
 
 STATIC_INLINE void