[Mesa-dev] [PATCH 1/2] nv50/ir: optimize signed integer modulo by pow-of-2

[Ilia Mirkin]

It's common to use signed int modulo in GLSL. As it happens, the GLSL
specs allow the result to be undefined, but that seems fairly
surprising. It's not that much more effort to get it right, at least for
positive modulo operators.

Signed-off-by: Ilia Mirkin <imirkin at alum.mit.edu>
---
 src/gallium/drivers/nouveau/codegen/nv50_ir.cpp    |  8 +------
 .../drivers/nouveau/codegen/nv50_ir_peephole.cpp   | 27 +++++++++++++++++++---
 2 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/src/gallium/drivers/nouveau/codegen/nv50_ir.cpp b/src/gallium/drivers/nouveau/codegen/nv50_ir.cpp
index 4076177e56d..657784163b3 100644
--- a/src/gallium/drivers/nouveau/codegen/nv50_ir.cpp
+++ b/src/gallium/drivers/nouveau/codegen/nv50_ir.cpp
@@ -429,13 +429,7 @@ ImmediateValue::isNegative() const
 bool
 ImmediateValue::isPow2() const
 {
-   switch (reg.type) {
-   case TYPE_U8:
-   case TYPE_U16:
-   case TYPE_U32: return util_is_power_of_two(reg.data.u32);
-   default:
-      return false;
-   }
+   return util_is_power_of_two(reg.data.u32);
 }
 
 void
diff --git a/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp b/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp
index 7e4e193e3d2..fabac662e7f 100644
--- a/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp
+++ b/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp
@@ -1163,10 +1163,31 @@ ConstantFolding::opnd(Instruction *i, ImmediateValue &imm0, int s)
       break;
 
    case OP_MOD:
-      if (i->sType == TYPE_U32 && imm0.isPow2()) {
+      if (s == 1 && imm0.isPow2()) {
          bld.setPosition(i, false);
-         i->op = OP_AND;
-         i->setSrc(1, bld.loadImm(NULL, imm0.reg.data.u32 - 1));
+         if (i->sType == TYPE_U32) {
+            i->op = OP_AND;
+            i->setSrc(1, bld.loadImm(NULL, imm0.reg.data.u32 - 1));
+         } else if (i->sType == TYPE_S32) {
+            // Do it on the absolute value of the input, and then restore the
+            // sign. The only odd case is MIN_INT, but that should work out
+            // as well, since MIN_INT mod any power of 2 is 0.
+            //
+            // Technically we don't have to do any of this since MOD is
+            // undefined with negative arguments in GLSL, but this seems like
+            // the nice thing to do.
+            Value *abs = bld.mkOp1v(OP_ABS, TYPE_S32, bld.getSSA(), i->getSrc(0));
+            Value *neg, *v1, *v2;
+            bld.mkCmp(OP_SET, CC_LT, TYPE_S32, (neg = bld.getSSA(1, FILE_FLAGS)), TYPE_S32, i->getSrc(0), bld.loadImm(NULL, 0));
+            Value *mod = bld.mkOp2v(OP_AND, TYPE_U32, bld.getSSA(), abs, bld.loadImm(NULL, imm0.reg.data.u32 - 1));
+            bld.mkOp1(OP_NEG, TYPE_S32, (v1 = bld.getSSA()), mod)
+               ->setPredicate(CC_P, neg);
+            bld.mkOp1(OP_MOV, TYPE_S32, (v2 = bld.getSSA()), mod)
+               ->setPredicate(CC_NOT_P, neg);
+            newi = bld.mkOp2(OP_UNION, TYPE_S32, i->getDef(0), v1, v2);
+
+            delete_Instruction(prog, i);
+         }
       }
       break;
 
-- 
2.13.6