Reorder branches in FIX:QUOTIENT/FIX:REMAINDER code on amd64.

Make the negative numerator case a forward branch so it will be
statically predicted not-taken.

diff --git a/src/compiler/machines/x86-64/rulfix.scm b/src/compiler/machines/x86-64/rulfix.scm
index 5f0ac12f60c1114cec2f79802d6aa383e2bd3d11..1dd9306662afb9451737b17e6d4ddadb87d56e95 100644 (file)
--- a/src/compiler/machines/x86-64/rulfix.scm
+++ b/src/compiler/machines/x86-64/rulfix.scm
@@ -600,14 +600,19 @@ USA.
          ((integer-power-of-2? (if (negative? n) (- 0 n) n))
           =>
           (lambda (expt-of-2)
-            (let ((label (generate-label 'QUO-SHIFT))
-                  (absn (if (negative? n) (- 0 n) n)))
+            (let ((if-negative (generate-label 'QUO-NEGATIVE))
+                  (merge (generate-label 'QUO-SHIFT)))
               (LAP (CMP Q ,target (& 0))
-                   (JGE B (@PCR ,label))
-                   ,@(with-unsigned-immediate-operand (* (- absn 1) fixnum-1)
+                   ;; Forward branch for the negative case so the
+                   ;; static prediction is not-taken.
+                   (JL B (@PCR ,if-negative))
+                   (JMP (@PCR ,merge))
+                  (LABEL ,if-negative)
+                   ,@(with-unsigned-immediate-operand
+                         (* (- (abs n) 1) fixnum-1)
                        (lambda (operand)
                          (LAP (ADD Q ,target ,operand))))
-                   (LABEL ,label)
+                  (LABEL ,merge)
                    (SAR Q ,target (&U ,expt-of-2))
                    ,@(word->fixnum target)
                    ,@(if (negative? n)
@@ -626,7 +631,8 @@ USA.
             (load-fixnum-constant 0 target))
            ((integer-power-of-2? n)
             (let ((sign (temporary-register-reference))
-                  (label (generate-label 'REM-MERGE)))
+                  (if-negative (generate-label 'REM-NEGATIVE))
+                  (merge (generate-label 'REM-MERGE)))
               ;; There is some hair here to deal with immediates that
               ;; don't fit in 32 bits, and reusing a temporary
               ;; register to store them.
@@ -644,12 +650,14 @@ USA.
                   (LAP (MOV Q ,sign ,target)
                        ,@prefix:n-1
                        (AND Q ,target ,operand:n-1)
-                       (JZ B (@PCR ,label))
+                       (JNZ B (@PCR ,if-negative))
+                       (JMP (@PCR ,merge))
+                      (LABEL ,if-negative)
                        (SAR Q ,sign (&U ,(-1+ scheme-object-width)))
                        ,@prefix:-n
                        (AND Q ,sign ,operand:-n)
                        (OR Q ,target ,sign)
-                       (LABEL ,label))))))
+                      (LABEL ,merge))))))
            (else
             (error "Fixnum-remainder/constant: Bad value" n))))))
 \f