Messages in this thread Patch in this message | | | From | Nadav Amit <> | Subject | [RFC 7/8] ilog2: preventing compiler distortion due to big condition | Date | Tue, 15 May 2018 07:11:14 -0700 |
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There are several places in the kernel in which there is a condition that is based on whether the input is known to be constant in compilation time. If it is, there are complex computations, which only take place during compilation time.
Although this scheme works correctly, when GCC computes the expected cost of this code in time and size, it disregards the fact that the computations of the "constant" case will not take place during runtime for the non-constant case. The cost of these functions is considered to be much higher. As a result, inline and branching decisions of the compiler are distorted. Specifically, functions are less likely to be inlined due to their preserved big size and execution time.
One of this cases is ilog2() which performs a complicated condition for constant inputs.
The solution is to use __builtin_choose_expr() to detect whether the input is constant instead of a C condition. GCC evaluates the builtin earlier, which allows it to improve code-generation decisions.
This patch allows to inline functions such as tbl_size().
text data bss dec hex filename 18148888 10064016 2936832 31149736 1db4ea8 ./vmlinux before 18149165 10064176 2936832 31150173 1db505d ./vmlinux after (+437)
Static text symbols: Before: 39650 After: 39643 (-7)
Cc: Randy Dunlap <rdunlap@infradead.org> Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Nadav Amit <namit@vmware.com> --- include/linux/log2.h | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/include/linux/log2.h b/include/linux/log2.h index 41a1ae010993..aa4dac874339 100644 --- a/include/linux/log2.h +++ b/include/linux/log2.h @@ -83,7 +83,8 @@ unsigned long __rounddown_pow_of_two(unsigned long n) */ #define ilog2(n) \ ( \ - __builtin_constant_p(n) ? ( \ + __builtin_choose_expr( \ + __builtin_constant_p(n), ( \ (n) < 2 ? 0 : \ (n) & (1ULL << 63) ? 63 : \ (n) & (1ULL << 62) ? 62 : \ @@ -147,10 +148,10 @@ unsigned long __rounddown_pow_of_two(unsigned long n) (n) & (1ULL << 4) ? 4 : \ (n) & (1ULL << 3) ? 3 : \ (n) & (1ULL << 2) ? 2 : \ - 1 ) : \ - (sizeof(n) <= 4) ? \ - __ilog2_u32(n) : \ - __ilog2_u64(n) \ + 1), \ + (sizeof(n) <= 4) ? \ + __ilog2_u32(n) : \ + __ilog2_u64(n)) \ ) /** -- 2.17.0
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