nir: Add partial redundancy elimination for compares

This pass attempts to dectect code sequences like

    if (x < y) {
        z = y - x;
	...
    }

and replace them with sequences like

    t = x - y;
    if (t < 0) {
        z = -t;
	...
    }

On architectures where the subtract can generate the flags used by the
if-statement, this saves an instruction.  It's also possible that moving
an instruction out of the if-statement will allow
nir_opt_peephole_select to convert the whole thing to a bcsel.

Currently only floating point compares and adds are supported.  Adding
support for integer will be a challenge due to integer overflow.  There
are a couple possible solutions, but they may not apply to all
architectures.

v2: Fix a typo in the commit message and a couple typos in comments.
Fix possible NULL pointer deref from result of push_block().  Add
missing (-A + B) case.  Suggested by Caio.

v3: Fix is_not_const_zero to work correctly with types other than
nir_type_float32.  Suggested by Ken.

v4: Add some comments explaining how this works.  Suggested by Ken.

Reviewed-by: Kenneth Graunke <[email protected]>

1 files changed, 27 insertions, 0 deletions

diff --git a/src/compiler/nir/nir_search_helpers.h b/src/compiler/nir/nir_search_helpers.h
index 456de81e175..1624508993d 100644
--- a/src/compiler/nir/nir_search_helpers.h
+++ b/src/compiler/nir/nir_search_helpers.h
@@ -110,6 +110,33 @@ is_zero_to_one(nir_alu_instr *instr, unsigned src, unsigned num_components,
 }
 
 static inline bool
+is_not_const_zero(nir_alu_instr *instr, unsigned src, unsigned num_components,
+                  const uint8_t *swizzle)
+{
+   if (nir_src_as_const_value(instr->src[src].src) == NULL)
+      return true;
+
+   for (unsigned i = 0; i < num_components; i++) {
+      switch (nir_op_infos[instr->op].input_types[src]) {
+      case nir_type_float:
+         if (nir_src_comp_as_float(instr->src[src].src, swizzle[i]) == 0.0)
+            return false;
+         break;
+      case nir_type_bool:
+      case nir_type_int:
+      case nir_type_uint:
+         if (nir_src_comp_as_uint(instr->src[src].src, swizzle[i]) == 0)
+            return false;
+         break;
+      default:
+         return false;
+      }
+   }
+
+   return true;
+}
+
+static inline bool
 is_not_const(nir_alu_instr *instr, unsigned src, UNUSED unsigned num_components,
              UNUSED const uint8_t *swizzle)
 {