Hexagon (target/hexagon) Short-circuit packet register writes

In certain cases, we can avoid the overhead of writing to hex_new_value
and write directly to hex_gpr.  We add need_commit field to DisasContext
indicating if the end-of-packet commit is needed.  If it is not needed,
get_result_gpr() and get_result_gpr_pair() can return hex_gpr.

We pass the ctx->need_commit to helpers when needed.

Finally, we can early-exit from gen_reg_writes during packet commit.

There are a few instructions whose semantics write to the result before
reading all the inputs.  Therefore, the idef-parser generated code is
incompatible with short-circuit.  We tell idef-parser to skip them.

For debugging purposes, we add a cpu property to turn off short-circuit.
When the short-circuit property is false, we skip the analysis and force
the end-of-packet commit.

Here's a simple example of the TCG generated for
0x004000b4:  0x7800c020 {       R0 = #0x1 }

BEFORE:
 ---- 004000b4
 movi_i32 new_r0,$0x1
 mov_i32 r0,new_r0

AFTER:
 ---- 004000b4
 movi_i32 r0,$0x1

This patch reintroduces a use of check_for_attrib, so we remove the
G_GNUC_UNUSED added earlier in this series.

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Brian Cain <bcain@quicinc.com>
Message-Id: <20230427230012.3800327-12-tsimpson@quicinc.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/target/hexagon/gen_tcg_funcs.py b/target/hexagon/gen_tcg_funcs.py
index d9ccbe63f6..0e45d43685 100755
--- a/target/hexagon/gen_tcg_funcs.py
+++ b/target/hexagon/gen_tcg_funcs.py
@@ -550,6 +550,9 @@ def gen_tcg_func(f, tag, regs, imms):
         if hex_common.need_pkt_has_multi_cof(tag):
             f.write("    TCGv pkt_has_multi_cof = ")
             f.write("tcg_constant_tl(ctx->pkt->pkt_has_multi_cof);\n")
+        if hex_common.need_pkt_need_commit(tag):
+            f.write("    TCGv pkt_need_commit = ")
+            f.write("tcg_constant_tl(ctx->need_commit);\n")
         if hex_common.need_part1(tag):
             f.write("    TCGv part1 = tcg_constant_tl(insn->part1);\n")
         if hex_common.need_slot(tag):
@@ -596,6 +599,8 @@ def gen_tcg_func(f, tag, regs, imms):
 
         if hex_common.need_pkt_has_multi_cof(tag):
             f.write(", pkt_has_multi_cof")
+        if hex_common.need_pkt_need_commit(tag):
+            f.write(", pkt_need_commit")
         if hex_common.need_PC(tag):
             f.write(", PC")
         if hex_common.helper_needs_next_PC(tag):