[PATCH] LoongArch: Set default alignment for functions and labels with -mtune

[Xi Ruoyao <xry111@xry111.site>]

The LA464 micro-architecture is sensitive to alignment of code.  The
Loongson team has benchmarked various combinations of function, the
results [1] show that 16-byte label alignment together with 32-byte
function alignment gives best results in terms of SPEC score.

Add a mtune-based table-driven mechanism to set the default of
-falign-{functions,labels}.  As LA464 is the first (and the only for
now) uarch supported by GCC, the same setting is also used for
the "generic" -mtune=loongarch64.  In the future we may set different
settings for LA{2,3,6}64 once we add the support for them.

Bootstrapped and regtested on loongarch64-linux-gnu.  Ok for trunk?

gcc/ChangeLog:

	* config/loongarch/loongarch-tune.h (loongarch_align): New
	struct.
	* config/loongarch/loongarch-def.h (loongarch_cpu_align): New
	array.
	* config/loongarch/loongarch-def.c (loongarch_cpu_align): Define
	the array.
	* config/loongarch/loongarch.cc
	(loongarch_option_override_internal): Set the value of
	-falign-functions= if -falign-functions is enabled but no value
	is given.  Likewise for -falign-labels=.
---
 gcc/config/loongarch/loongarch-def.c  | 12 ++++++++++++
 gcc/config/loongarch/loongarch-def.h  |  1 +
 gcc/config/loongarch/loongarch-tune.h |  8 ++++++++
 gcc/config/loongarch/loongarch.cc     |  6 ++++++
 4 files changed, 27 insertions(+)

diff --git a/gcc/config/loongarch/loongarch-def.c b/gcc/config/loongarch/loongarch-def.c
index fc4ebbefede..6729c857f7c 100644
--- a/gcc/config/loongarch/loongarch-def.c
+++ b/gcc/config/loongarch/loongarch-def.c
@@ -72,6 +72,18 @@ loongarch_cpu_cache[N_TUNE_TYPES] = {
   },
 };
 
+struct loongarch_align
+loongarch_cpu_align[N_TUNE_TYPES] = {
+  [CPU_LOONGARCH64] = {
+    .function = "32",
+    .label = "16",
+  },
+  [CPU_LA464] = {
+    .function = "32",
+    .label = "16",
+  },
+};
+
 /* The following properties cannot be looked up directly using "cpucfg".
  So it is necessary to provide a default value for "unknown native"
  tune targets (i.e. -mtune=native while PRID does not correspond to
diff --git a/gcc/config/loongarch/loongarch-def.h b/gcc/config/loongarch/loongarch-def.h
index 778b1409956..fb8bb88eb52 100644
--- a/gcc/config/loongarch/loongarch-def.h
+++ b/gcc/config/loongarch/loongarch-def.h
@@ -144,6 +144,7 @@ extern int loongarch_cpu_issue_rate[];
 extern int loongarch_cpu_multipass_dfa_lookahead[];
 
 extern struct loongarch_cache loongarch_cpu_cache[];
+extern struct loongarch_align loongarch_cpu_align[];
 extern struct loongarch_rtx_cost_data loongarch_cpu_rtx_cost_data[];
 
 #ifdef __cplusplus
diff --git a/gcc/config/loongarch/loongarch-tune.h b/gcc/config/loongarch/loongarch-tune.h
index ba31c4f08c3..5c03262daff 100644
--- a/gcc/config/loongarch/loongarch-tune.h
+++ b/gcc/config/loongarch/loongarch-tune.h
@@ -48,4 +48,12 @@ struct loongarch_cache {
     int simultaneous_prefetches; /* number of parallel prefetch */
 };
 
+/* Alignment for functions and labels for best performance.  For new uarchs
+   the value should be measured via benchmarking.  See the documentation for
+   -falign-functions and -falign-labels in invoke.texi for the format.  */
+struct loongarch_align {
+  const char *function;	/* default value for -falign-functions */
+  const char *label;	/* default value for -falign-labels */
+};
+
 #endif /* LOONGARCH_TUNE_H */
diff --git a/gcc/config/loongarch/loongarch.cc b/gcc/config/loongarch/loongarch.cc
index eb73d11b869..5b8b93eb24b 100644
--- a/gcc/config/loongarch/loongarch.cc
+++ b/gcc/config/loongarch/loongarch.cc
@@ -6249,6 +6249,12 @@ loongarch_option_override_internal (struct gcc_options *opts)
       && !opts->x_optimize_size)
     opts->x_flag_prefetch_loop_arrays = 1;
 
+  if (opts->x_flag_align_functions && !opts->x_str_align_functions)
+    opts->x_str_align_functions = loongarch_cpu_align[LARCH_ACTUAL_TUNE].function;
+
+  if (opts->x_flag_align_labels && !opts->x_str_align_labels)
+    opts->x_str_align_labels = loongarch_cpu_align[LARCH_ACTUAL_TUNE].label;
+
   if (TARGET_DIRECT_EXTERN_ACCESS && flag_shlib)
     error ("%qs cannot be used for compiling a shared library",
 	   "-mdirect-extern-access");
-- 
2.41.0