您應該標記函數static
,以便編譯器知道它們對於該翻譯單元是本地的。
沒有static
編譯器不能假設(禁止LTO/WPA)該函數只被調用一次,因此不太可能將其內聯。
演示使用LLVM Try Out頁面。
這就是說,代碼的可讀性首先,微優化(並且這樣調整是微優化)應該僅在性能測量之後出現。
例子:
#include <cstdio>
static void foo(int i) {
int m = i % 3;
printf("%d %d", i, m);
}
int main(int argc, char* argv[]) {
for (int i = 0; i != argc; ++i) {
foo(i);
}
}
產生一個與static
:
; ModuleID = '/tmp/webcompile/_27689_0.bc'
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-unknown-linux-gnu"
@.str = private constant [6 x i8] c"%d %d\00" ; <[6 x i8]*> [#uses=1]
define i32 @main(i32 %argc, i8** nocapture %argv) nounwind {
entry:
%cmp4 = icmp eq i32 %argc, 0 ; <i1> [#uses=1]
br i1 %cmp4, label %for.end, label %for.body
for.body: ; preds = %for.body, %entry
%0 = phi i32 [ %inc, %for.body ], [ 0, %entry ] ; <i32> [#uses=3]
%rem.i = srem i32 %0, 3 ; <i32> [#uses=1]
%call.i = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([6 x i8]* @.str, i64 0, i64 0), i32 %0, i32 %rem.i) nounwind ; <i32> [#uses=0]
%inc = add nsw i32 %0, 1 ; <i32> [#uses=2]
%exitcond = icmp eq i32 %inc, %argc ; <i1> [#uses=1]
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body, %entry
ret i32 0
}
declare i32 @printf(i8* nocapture, ...) nounwind
沒有static
:
; ModuleID = '/tmp/webcompile/_27859_0.bc'
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-unknown-linux-gnu"
@.str = private constant [6 x i8] c"%d %d\00" ; <[6 x i8]*> [#uses=1]
define void @foo(int)(i32 %i) nounwind {
entry:
%rem = srem i32 %i, 3 ; <i32> [#uses=1]
%call = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([6 x i8]* @.str, i64 0, i64 0), i32 %i, i32 %rem) ; <i32> [#uses=0]
ret void
}
declare i32 @printf(i8* nocapture, ...) nounwind
define i32 @main(i32 %argc, i8** nocapture %argv) nounwind {
entry:
%cmp4 = icmp eq i32 %argc, 0 ; <i1> [#uses=1]
br i1 %cmp4, label %for.end, label %for.body
for.body: ; preds = %for.body, %entry
%0 = phi i32 [ %inc, %for.body ], [ 0, %entry ] ; <i32> [#uses=3]
%rem.i = srem i32 %0, 3 ; <i32> [#uses=1]
%call.i = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([6 x i8]* @.str, i64 0, i64 0), i32 %0, i32 %rem.i) nounwind ; <i32> [#uses=0]
%inc = add nsw i32 %0, 1 ; <i32> [#uses=2]
%exitcond = icmp eq i32 %inc, %argc ; <i1> [#uses=1]
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body, %entry
ret i32 0
}
也許,也許不是。編譯器程序員可能是你最好的選擇,根據你使用的編譯器來決定。 – DumbCoder
我想知道內聯點會是什麼......電話很便宜。 – 2011-08-17 16:45:18
這些都不是一個循環?您希望從避免幾次函數調用的開銷中獲得多少時間?一納秒? – UncleBens