nir/gcm: Add global value numbering support

Unlike the current CSE pass, global value numbering is capable of detecting
common values even if one does not dominate the other.  For instance, in
you have

if (...) {
   ssa_1 = ssa_0 + 7;
   /* use ssa_1 */
} else {
   ssa_2 = ssa_0 + 7;
   /* use ssa_2 */
}

Global value numbering doesn't care about dominance relationships so it
figures out that ssa_1 and ssa_2 are the same and converts this to

if (...) {
   ssa_1 = ssa_0 + 7;
   /* use ssa_1 */
} else {
   /* use ssa_1 */
}

Obviously, we just broke SSA form which is bad.  Global code motion,
however, will repair this for us by turning this into

ssa_1 = ssa_0 + 7;
if (...) {
   /* use ssa_1 */
} else {
   /* use ssa_1 */
}

This intended to eventually mostly replace CSE.  However, conventional CSE
may still be useful because it's less of a scorched-earth approach and
doesn't require GCM.  This makes it a bit more appropriate for use as a
clean-up in a late optimization run.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
index c1cf94001f6540540f7d9d9262f43fa8c713b891..ff7c422a6ea087ab8db57a58acd2fedf6b2315f5 100644 (file)
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -2587,7 +2587,7 @@ bool nir_opt_dce(nir_shader *shader);
 
 bool nir_opt_dead_cf(nir_shader *shader);
 
-void nir_opt_gcm(nir_shader *shader);
+bool nir_opt_gcm(nir_shader *shader, bool value_number);
 
 bool nir_opt_peephole_select(nir_shader *shader);
 

diff --git a/src/compiler/nir/nir_opt_gcm.c b/src/compiler/nir/nir_opt_gcm.c
index 02a93489ebaf97e9eeff83ac8d62654c9d56ac39..77eb8e6da5ab6f0a6983ad0df8e65239dec975d4 100644 (file)
--- a/src/compiler/nir/nir_opt_gcm.c
+++ b/src/compiler/nir/nir_opt_gcm.c
@@ -26,6 +26,7 @@
  */
 
 #include "nir.h"
+#include "nir_instr_set.h"
 
 /*
  * Implements Global Code Motion.  A description of GCM can be found in
@@ -451,8 +452,8 @@ gcm_place_instr(nir_instr *instr, struct gcm_state *state)
    block_info->last_instr = instr;
 }
 
-static void
-opt_gcm_impl(nir_function_impl *impl)
+static bool
+opt_gcm_impl(nir_function_impl *impl, bool value_number)
 {
    struct gcm_state state;
 
@@ -470,6 +471,18 @@ opt_gcm_impl(nir_function_impl *impl)
       gcm_pin_instructions_block(block, &state);
    }
 
+   bool progress = false;
+   if (value_number) {
+      struct set *gvn_set = nir_instr_set_create(NULL);
+      foreach_list_typed_safe(nir_instr, instr, node, &state.instrs) {
+         if (nir_instr_set_add_or_rewrite(gvn_set, instr)) {
+            nir_instr_remove(instr);
+            progress = true;
+         }
+      }
+      nir_instr_set_destroy(gvn_set);
+   }
+
    foreach_list_typed(nir_instr, instr, node, &state.instrs)
       gcm_schedule_early_instr(instr, &state);
 
@@ -486,13 +499,19 @@ opt_gcm_impl(nir_function_impl *impl)
 
    nir_metadata_preserve(impl, nir_metadata_block_index |
                                nir_metadata_dominance);
+
+   return progress;
 }
 
-void
-nir_opt_gcm(nir_shader *shader)
+bool
+nir_opt_gcm(nir_shader *shader, bool value_number)
 {
+   bool progress = false;
+
    nir_foreach_function(function, shader) {
       if (function->impl)
-         opt_gcm_impl(function->impl);
+         progress |= opt_gcm_impl(function->impl, value_number);
    }
+
+   return progress;
 }