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Subject[for-next][PATCH 1/5] tracing: Simplify "if" macro code
From: Linus Torvalds <torvalds@linux-foundation.org>

Peter Zijlstra noticed that with CONFIG_PROFILE_ALL_BRANCHES, the "if"
macro converts the conditional to an array index. This can cause GCC
to create horrible code. When there are nested ifs, the generated code
uses register values to encode branching decisions.

Josh Poimboeuf found that replacing the define "if" macro from using
the condition as an array index and incrementing the branch statics
with an if statement itself, reduced the asm complexity and shrinks the
generated code quite a bit.

But this can be simplified even further by replacing the internal if
statement with a ternary operator.

Link: https://lkml.kernel.org/r/20190307174802.46fmpysxyo35hh43@treble
Link: http://lkml.kernel.org/r/CAHk-=wiALN3jRuzARpwThN62iKd476Xj-uom+YnLZ4=eqcz7xQ@mail.gmail.com

Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
include/linux/compiler.h | 35 ++++++++++++++++++-----------------
1 file changed, 18 insertions(+), 17 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 445348facea9..8aaf7cd026b0 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -53,23 +53,24 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
* "Define 'is'", Bill Clinton
* "Define 'if'", Steven Rostedt
*/
-#define if(cond, ...) __trace_if( (cond , ## __VA_ARGS__) )
-#define __trace_if(cond) \
- if (__builtin_constant_p(!!(cond)) ? !!(cond) : \
- ({ \
- int ______r; \
- static struct ftrace_branch_data \
- __aligned(4) \
- __section("_ftrace_branch") \
- ______f = { \
- .func = __func__, \
- .file = __FILE__, \
- .line = __LINE__, \
- }; \
- ______r = !!(cond); \
- ______f.miss_hit[______r]++; \
- ______r; \
- }))
+#define if(cond, ...) if ( __trace_if_var( !!(cond , ## __VA_ARGS__) ) )
+
+#define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond))
+
+#define __trace_if_value(cond) ({ \
+ static struct ftrace_branch_data \
+ __aligned(4) \
+ __section("_ftrace_branch") \
+ __if_trace = { \
+ .func = __func__, \
+ .file = __FILE__, \
+ .line = __LINE__, \
+ }; \
+ (cond) ? \
+ (__if_trace.miss_hit[1]++,1) : \
+ (__if_trace.miss_hit[0]++,0); \
+})
+
#endif /* CONFIG_PROFILE_ALL_BRANCHES */

#else
--
2.20.1

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