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| author | Xavier Leroy <xavier.leroy@college-de-france.fr> | 2019-05-31 19:15:19 +0200 |
|---|---|---|
| committer | Xavier Leroy <xavierleroy@users.noreply.github.com> | 2019-06-06 10:58:13 +0200 |
| commit | 8e3a73448c5ddfa4be3871d7f4fd80281a7549f4 (patch) | |
| tree | 1d2faaa30db1d4c8d7e236f7ddfbd145d778e6e3 /driver | |
| parent | 0025c4db576dbc174946b7adb83d4e0b81ce4b5f (diff) | |
| download | compcert-8e3a73448c5ddfa4be3871d7f4fd80281a7549f4.tar.gz compcert-8e3a73448c5ddfa4be3871d7f4fd80281a7549f4.zip | |
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.
Diffstat (limited to 'driver')
| -rw-r--r-- | driver/Clflags.ml | 2 | ||||
| -rw-r--r-- | driver/Driver.ml | 10 |
2 files changed, 10 insertions, 2 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 diff --git a/driver/Driver.ml b/driver/Driver.ml index 50f14d13..84392ef6 100644 --- a/driver/Driver.ml +++ b/driver/Driver.ml @@ -196,6 +196,9 @@ Processing options: -finline Perform inlining of functions [on] -finline-functions-called-once Integrate functions only required by their single caller [on] + -fif-conversion Perform if-conversion (generation of conditional moves) [on] + -ffavor-branchless Favor the generation of branch-free instruction sequences, + even when possibly more costly than the default [off] Code generation options: (use -fno-<opt> to turn off -f<opt>) -ffpu Use FP registers for some integer operations [on] -fsmall-data <n> Set maximal size <n> for allocation in small data area @@ -250,7 +253,8 @@ let dump_mnemonics destfile = exit 0 let optimization_options = [ - option_ftailcalls; option_fconstprop; option_fcse; option_fredundancy; option_finline_functions_called_once; + option_ftailcalls; option_fifconversion; option_fconstprop; option_fcse; + option_fredundancy; option_finline_functions_called_once; ] let set_all opts () = List.iter (fun r -> r := true) opts @@ -301,7 +305,8 @@ let cmdline_actions = Exact "-Os", Set option_Osize; Exact "-fsmall-data", Integer(fun n -> option_small_data := n); Exact "-fsmall-const", Integer(fun n -> option_small_const := n); - Exact "-ffloat-const-prop", Integer(fun n -> option_ffloatconstprop := n); + Exact "-ffloat-const-prop", Integer(fun n -> option_ffloatconstprop := n); + Exact "-ffavor-branchless", Set option_ffavor_branchless; Exact "-falign-functions", Integer(fun n -> check_align n; option_falignfunctions := Some n); Exact "-falign-branch-targets", Integer(fun n -> check_align n; option_falignbranchtargets := n); Exact "-falign-cond-branches", Integer(fun n -> check_align n; option_faligncondbranchs := n);] @ @@ -364,6 +369,7 @@ let cmdline_actions = (* Optimization options *) (* -f options: come in -f and -fno- variants *) @ f_opt "tailcalls" option_ftailcalls + @ f_opt "if-conversion" option_fifconversion @ f_opt "const-prop" option_fconstprop @ f_opt "cse" option_fcse @ f_opt "redundancy" option_fredundancy |