If-conversion optimization

Extends the instruction selection pass with an if-conversion optimization:
some if/then/else statements are converted into "select" operations,
which in turn can be compiled down to branchless instruction sequences
if the target architecture supports them.

The statements that are converted are of the form

        if (cond) { x = a1; } else { x = a2; }
        if (cond) { x = a1; }
        if (cond) { /*skip*/; } else { x = a2; }

where a1, a2 are "safe" expressions, containing no operations that can
fail at run-time, such as memory loads or integer divisions.

A heuristic in backend/Selectionaux.ml controls when the optimization occurs,
depending on command-line flags and the complexity of the "then" and "else"
branches.

1 files changed, 2 insertions, 0 deletions

diff --git a/driver/Clflags.ml b/driver/Clflags.ml
index fc12863d..d27871ef 100644
--- a/driver/Clflags.ml
+++ b/driver/Clflags.ml
@@ -27,6 +27,8 @@ let option_ftailcalls = ref true
 let option_fconstprop = ref true
 let option_fcse = ref true
 let option_fredundancy = ref true
+let option_fifconversion = ref true
+let option_ffavor_branchless = ref false
 let option_falignfunctions = ref (None: int option)
 let option_falignbranchtargets = ref 0
 let option_faligncondbranchs = ref 0