Author: David Schneider <[email protected]>
Branch: extradoc
Changeset: r4306:f2d396882dde
Date: 2012-07-18 11:04 +0200
http://bitbucket.org/pypy/extradoc/changeset/f2d396882dde/
Log: clarify this point
diff --git a/talk/vmil2012/paper.tex b/talk/vmil2012/paper.tex
--- a/talk/vmil2012/paper.tex
+++ b/talk/vmil2012/paper.tex
@@ -203,16 +203,17 @@
\label{sec:Guards in the Backend}
Code generation consists of two passes over the lists of instructions, a
-backwards pass to calculate live ranges of IR-level variables \cfbolz{doesn't
the backward pass also remove dead instructions?} and a forward one
+backwards pass to calculate live ranges of IR-level variables and a forward one
to emit the instructions. During the forward pass IR-level variables are
-assigned to registers and stack locations by the register allocator according
to
-the requirements of the to be emitted instructions. Eviction/spilling is
+assigned to registers and stack locations by the register allocator according
+to the requirements of the to be emitted instructions. Eviction/spilling is
performed based on the live range information collected in the first pass. Each
IR instruction is transformed into one or more machine level instructions that
-implement the required semantics. Guards instructions are transformed into
-fast checks at the machine code level that verify the corresponding condition.
-In cases the value being checked by the guard is not used anywhere else the
-guard and the operation producing the value can merged, reducing even more the
+implement the required semantics, operations withouth side effects whose result
+is not used are not emitted. Guards instructions are transformed into fast
+checks at the machine code level that verify the corresponding condition. In
+cases the value being checked by the guard is not used anywhere else the guard
+and the operation producing the value can merged, reducing even more the
overhead of the guard.
Each guard in the IR has attached to it a list of the IR-variables required to
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