These macros complement and extend the existing do_it() macro.
Together, they streamline the process of optimizing short branchless
contitional sequences to support ARM, Thumb-2, and Thumb-1.
The inherent architecture limitations of Thumb-1 means that writing
assembly code is somewhat more tedious. And, while such code will run
unmodified in an ARM or Thumb-2 enfironment, it will lack one of the
key performance optimizations available there.
Initially, the first idea might be to split the an instruction sequence
with #ifdef(s): one path for Thumb-1 and the other for ARM/Thumb-2.
This could suffice if conditional execution optimizations were rare.
However, #ifdef(s) break flow of an algorithm and shift focus to the
architectural differences instead of the similarities. On functions
with a high percentage of conditional execution, it starts to become
attractive to split everything into distinct architecture-specific
function objects -- even when the underlying algorithm is identical.
Additionally, duplicated code and comments (whether an individual
operand, a line, or a larger block) become a future maintenance
liability if the two versions aren't kept in sync.
See code comments for limitations and expecated usage.
gcc/libgcc/ChangeLog:
2021-01-14 Daniel Engel <g...@danielengel.com>
(__HAVE_FEATURE_IT, IT): New macros.
---
libgcc/config/arm/lib1funcs.S | 68 +++++++++++++++++++++++++++++++++++
1 file changed, 68 insertions(+)
diff --git a/libgcc/config/arm/lib1funcs.S b/libgcc/config/arm/lib1funcs.S
index b8693be8e4f..1233b8c0992 100644
--- a/libgcc/config/arm/lib1funcs.S
+++ b/libgcc/config/arm/lib1funcs.S
@@ -230,6 +230,7 @@ LSYM(Lend_fde):
ARM and Thumb-2. However this is only supported by recent gas, so define
a set of macros to allow ARM code on older assemblers. */
#if defined(__thumb2__)
+#define __HAVE_FEATURE_IT
.macro do_it cond, suffix=""
it\suffix \cond
.endm
@@ -245,6 +246,9 @@ LSYM(Lend_fde):
\name \dest, \src1, \tmp
.endm
#else
+#if !defined(__thumb__)
+#define __HAVE_FEATURE_IT
+#endif
.macro do_it cond, suffix=""
.endm
.macro shift1 op, arg0, arg1, arg2
@@ -259,6 +263,70 @@ LSYM(Lend_fde):
#define COND(op1, op2, cond) op1 ## op2 ## cond
+
+/* The IT() macro streamlines the construction of short branchless contitional
+ sequences that support ARM, Thumb-2, and Thumb-1. It is intended as an
+ extension to the .do_it macro defined above. Code not written with the
+ intent to support Thumb-1 need not use IT().
+
+ IT()'s main advantage is the minimization of syntax differences. Unified
+ functions can support Thumb-1 without imposiing an undue performance
+ penalty on ARM and Thumb-2. Writing code without duplicate instructions
+ and operands keeps the high level function flow clearer and should reduce
+ the incidence of maintenance bugs.
+
+ Where conditional execution is supported by ARM and Thumb-2, the specified
+ instruction compiles with the conditional suffix 'c'.
+
+ Where Thumb-1 and v6m do not support IT, the given instruction compiles
+ with the standard unified syntax suffix "s", and a preceding branch
+ instruction is required to implement conditional behavior.
+
+ (Aside: The Thumb-1 "s"-suffix pattern is somewhat simplistic, since it
+ does not support 'cmp' or 'tst' with a non-"s" suffix. It also appends
+ "s" to 'mov' and 'add' with high register operands which are otherwise
+ legal on v6m. Use of IT() will result in a compiler error for all of
+ these exceptional cases, and a full #ifdef code split will be required.
+ However, it is unlikely that code written with Thumb-1 compatibility
+ in mind will use such patterns, so IT() still promises a good value.)
+
+ Typical if/then/else usage is:
+
+ #ifdef __HAVE_FEATURE_IT
+ // ARM and Thumb-2 'true' condition.
+ do_it c, tee
+ #else
+ // Thumb-1 'false' condition. This must be opposite the
+ // sense of the ARM and Thumb-2 condition, since the
+ // branch is taken to skip the 'true' instruction block.
+ b!c else_label
+ #endif
+
+ // Conditional 'true' execution for all compile modes.
+ IT(ins1,c) op1, op2
+ IT(ins2,c) op1, op2
+
+ #ifndef __HAVE_FEATURE_IT
+ // Thumb-1 branch to skip the 'else' instruction block.
+ // Omitted for if/then usage.
+ b end_label
+ #endif
+
+ else_label:
+ // Conditional 'false' execution for all compile modes.
+ // Omitted for if/then usage.
+ IT(ins3,!c) op1, op2
+ IT(ins4,!c) op1, op2
+
+ end_label:
+ // Unconditional execution resumes here.
+ */
+#ifdef __HAVE_FEATURE_IT
+ #define IT(ins,c) ins##c
+#else
+ #define IT(ins,c) ins##s
+#endif
+
#ifdef __ARM_EABI__
.macro ARM_LDIV0 name signed
cmp r0, #0
--
2.25.1
