intel/compiler: split is_partial_write() into two variants

This function is used in two different scenarios that for 32-bit
instructions are the same, but for 16-bit instructions are not.

One scenario is that in which we are working at a SIMD8 register
level and we need to know if a register is fully defined or written.
This is useful, for example, in the context of liveness analysis or
register allocation, where we work with units of registers.

The other scenario is that in which we want to know if an instruction
is writing a full scalar component or just some subset of it. This is
useful, for example, in the context of some optimization passes
like copy propagation.

For 32-bit instructions (or larger), a SIMD8 dispatch will always write
at least a full SIMD8 register (32B) if the write is not partial. The
function is_partial_write() checks this to determine if we have a partial
write. However, when we deal with 16-bit instructions, that logic disables
some optimizations that should be safe. For example, a SIMD8 16-bit MOV will
only update half of a SIMD register, but it is still a complete write of the
variable for a SIMD8 dispatch, so we should not prevent copy propagation in
this scenario because we don't write all 32 bytes in the SIMD register
or because the write starts at offset 16B (wehere we pack components Y or
W of 16-bit vectors).

This is a problem for SIMD8 executions (VS, TCS, TES, GS) of 16-bit
instructions, which lose a number of optimizations because of this, most
important of which is copy-propagation.

This patch splits is_partial_write() into is_partial_reg_write(), which
represents the current is_partial_write(), useful for things like
liveness analysis, and is_partial_var_write(), which considers
the dispatch size to check if we are writing a full variable (rather
than a full register) to decide if the write is partial or not, which
is what we really want in many optimization passes.

Then the patch goes on and rewrites all uses of is_partial_write() to use
one or the other version. Specifically, we use is_partial_var_write()
in the following places: copy propagation, cmod propagation, common
subexpression elimination, saturate propagation and sel peephole.

Notice that the semantics of is_partial_var_write() exactly match the
current implementation of is_partial_write() for anything that is
32-bit or larger, so no changes are expected for 32-bit instructions.

Tested against ~5000 tests involving 16-bit instructions in CTS produced
the following changes in instruction counts:

            Patched  |     Master    |    %    |
================================================
SIMD8  |    621,900  |    706,721    | -12.00% |
================================================
SIMD16 |     93,252  |     93,252    |   0.00% |
================================================

As expected, the change only affects SIMD8 dispatches.

Reviewed-by: Topi Pohjolainen <[email protected]>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/intel/compiler/brw_ir_fs.h b/src/intel/compiler/brw_ir_fs.h
index 56a4bdc6e52..87e03258d93 100644
--- a/src/intel/compiler/brw_ir_fs.h
+++ b/src/intel/compiler/brw_ir_fs.h
@@ -348,7 +348,8 @@ public:
    void resize_sources(uint8_t num_sources);
 
    bool is_send_from_grf() const;
-   bool is_partial_write() const;
+   bool is_partial_reg_write() const;
+   bool is_partial_var_write(unsigned dispatch_width) const;
    bool is_copy_payload(const brw::simple_allocator &grf_alloc) const;
    unsigned components_read(unsigned i) const;
    unsigned size_read(int arg) const;