diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
index a5c7e9346fd..6e3429f72e1 100644
--- a/src/intel/compiler/brw_fs.cpp
+++ b/src/intel/compiler/brw_fs.cpp
@@ -2549,6 +2549,9 @@ fs_visitor::opt_algebraic()
             assert(!inst->src[0].negate);
             const brw::fs_builder ibld(this, block, inst);
 
+            if (!inst->is_partial_write())
+               ibld.emit_undef_for_dst(inst);
+
             ibld.MOV(subscript(inst->dst, BRW_REGISTER_TYPE_F, 1),
                      subscript(inst->src[0], BRW_REGISTER_TYPE_F, 1));
             ibld.MOV(subscript(inst->dst, BRW_REGISTER_TYPE_F, 0),
@@ -2567,6 +2570,9 @@ fs_visitor::opt_algebraic()
             assert(!inst->src[0].negate);
             const brw::fs_builder ibld(this, block, inst);
 
+            if (!inst->is_partial_write())
+               ibld.emit_undef_for_dst(inst);
+
             ibld.MOV(subscript(inst->dst, BRW_REGISTER_TYPE_UD, 1),
                      subscript(inst->src[0], BRW_REGISTER_TYPE_UD, 1));
             ibld.MOV(subscript(inst->dst, BRW_REGISTER_TYPE_UD, 0),
@@ -2697,6 +2703,9 @@ fs_visitor::opt_algebraic()
             assert(!inst->src[1].abs && !inst->src[1].negate);
             const brw::fs_builder ibld(this, block, inst);
 
+            if (!inst->is_partial_write())
+               ibld.emit_undef_for_dst(inst);
+
             set_predicate(inst->predicate,
                           ibld.SEL(subscript(inst->dst, BRW_REGISTER_TYPE_UD, 0),
                                    subscript(inst->src[0], BRW_REGISTER_TYPE_UD, 0),
@@ -4107,6 +4116,7 @@ fs_visitor::lower_mul_qword_inst(fs_inst *inst, bblock_t *block)
                 subscript(inst->src[1], BRW_REGISTER_TYPE_UD, 0));
       ibld.MOV(bd_low, acc);
 
+      ibld.UNDEF(bd);
       ibld.MOV(subscript(bd, BRW_REGISTER_TYPE_UD, 0), bd_low);
       ibld.MOV(subscript(bd, BRW_REGISTER_TYPE_UD, 1), bd_high);
    }
@@ -4123,6 +4133,8 @@ fs_visitor::lower_mul_qword_inst(fs_inst *inst, bblock_t *block)
    if (devinfo->has_64bit_int) {
       ibld.MOV(inst->dst, bd);
    } else {
+      if (!inst->is_partial_write())
+         ibld.emit_undef_for_dst(inst);
       ibld.MOV(subscript(inst->dst, BRW_REGISTER_TYPE_UD, 0),
                subscript(bd, BRW_REGISTER_TYPE_UD, 0));
       ibld.MOV(subscript(inst->dst, BRW_REGISTER_TYPE_UD, 1),
@@ -5564,6 +5576,10 @@ fs_visitor::lower_find_live_channel()
        */
       fs_reg exec_mask(retype(brw_mask_reg(0), BRW_REGISTER_TYPE_UD));
 
+      const fs_builder ibld(this, block, inst);
+      if (!inst->is_partial_write())
+         ibld.emit_undef_for_dst(inst);
+
       const fs_builder ubld = bld.at(block, inst).exec_all().group(1, 0);
 
       /* ce0 doesn't consider the thread dispatch mask (DMask or VMask),
diff --git a/src/intel/compiler/brw_fs_builder.h b/src/intel/compiler/brw_fs_builder.h
index 7b4cdd726a2..d2d8d5e7ef0 100644
--- a/src/intel/compiler/brw_fs_builder.h
+++ b/src/intel/compiler/brw_fs_builder.h
@@ -565,6 +565,17 @@ namespace brw {
          }
       }
 
+      instruction *
+      emit_undef_for_dst(const instruction *old_inst) const
+      {
+         assert(old_inst->dst.file == VGRF);
+         instruction *inst = emit(SHADER_OPCODE_UNDEF,
+                                  retype(old_inst->dst, BRW_REGISTER_TYPE_UD));
+         inst->size_written = old_inst->size_written;
+
+         return inst;
+      }
+
       /**
        * Assorted arithmetic ops.
        * @{
@@ -785,7 +796,7 @@ namespace brw {
          assert(dst.offset % REG_SIZE == 0);
          instruction *inst = emit(SHADER_OPCODE_UNDEF,
                                   retype(dst, BRW_REGISTER_TYPE_UD));
-         inst->size_written = shader->alloc.sizes[dst.nr] * REG_SIZE;
+         inst->size_written = shader->alloc.sizes[dst.nr] * REG_SIZE - dst.offset;
 
          return inst;
       }
diff --git a/src/intel/compiler/brw_fs_lower_pack.cpp b/src/intel/compiler/brw_fs_lower_pack.cpp
index ac7b61de6f0..0b0f9417513 100644
--- a/src/intel/compiler/brw_fs_lower_pack.cpp
+++ b/src/intel/compiler/brw_fs_lower_pack.cpp
@@ -41,6 +41,13 @@ fs_visitor::lower_pack()
       fs_reg dst = inst->dst;
 
       const fs_builder ibld(this, block, inst);
+      /* The lowering generates 2 instructions for what was previously 1. This
+       * can trick the IR to believe we're doing partial writes, but the
+       * register is actually fully written. Mark it as undef to help the IR
+       * reduce the liveness of the register.
+       */
+      if (!inst->is_partial_write())
+         ibld.emit_undef_for_dst(inst);
       for (unsigned i = 0; i < inst->sources; i++)
          ibld.MOV(subscript(dst, inst->src[i].type, i), inst->src[i]);