使用LLVM编写的B编译器示例
项目描述
此存储库包含我作为2015年寒假项目编写的实验性B编译器。
安装和入门
llvmlite绑定需要（截至撰写时）LLVM版本3.6。请确保正确设置LLVM_CONFIG环境变量。
在基于Debian/Ubuntu的Linux上
$ sudo apt install libedit-dev llvm-dev-3.6 $ export LLVM_CONFIG=llvm-config-3.6
在OSX上
$ brew install llvm $ export LLVM_CONFIG=/usr/local/opt/llvm/bin/llvm-config
然后通过pip安装
$ pip install git+https://github.com/rjw57/rbc
如果需要，开发者可以手动克隆存储库
$ git clone git://github.com/rjw57/rbc $ cd /path/to/rbc; pip install -e .
然后可以通过rbc工具编译B代码
$ echo 'main(){ extrn putstr; putstr("hello!*n"); }' >greet.b
$ rbc -o greet greet.b
$ ./greet
hello!
示例
计算e
以下示例取自B教程
/* calce.b:
The following program will calculate the constant e-2 to about
4000 decimal digits, and print it 50 characters to the line in
groups of 5 characters. The method is simple output conversion
of the expansion
1/2! + 1/3! + ... = .111....
where the bases of the digits are 2, 3, 4, . . . */
main() {
extrn putchar, n, v;
auto i, c, col, a;
i = col = 0;
while(i<n)
v[i++] = 1;
while(col<2*n) {
a = n+1 ;
c = i = 0;
while (i<n) {
c =+ v[i] *10;
v[i++] = c%a;
c =/ a--;
}
putchar(c+'0');
if(!(++col%5))
putchar(col%50?' ': '*n');
}
putchar('*n*n');
}
v[2000];
n 2000;
将此代码保存到calce.b。编译并运行
$ rbc calce.b && ./a.out | head 71828 18284 59045 23536 02874 71352 66249 77572 47093 69995 95749 66967 62772 40766 30353 54759 45713 82178 52516 64274 27466 39193 20030 59921 81741 35966 29043 57290 03342 95260 59563 07381 32328 62794 34907 63233 82988 07531 95251 01901 15738 34187 93070 21540 89149 93488 41675 09244 76146 06680 82264 80016 84774 11853 74234 54424 37107 53907 77449 92069 55170 27618 38606 26133 13845 83000 75204 49338 26560 29760 67371 13200 70932 87091 27443 74704 72306 96977 20931 01416 92836 81902 55151 08657 46377 21112 52389 78442 50569 53696 77078 54499 69967 94686 44549 05987 93163 68892 30098 79312
阶乘
以下示例显示了编译器如何优化递归
$ echo 'fact(n) return (n==0?1:n*fact(n-1));' >fact.b
$ rbc --emit-llvm -s fact.b
$ cat fact.ll
; ModuleID = 'fact.b'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"
; Function Attrs: nounwind readnone
define i64 @b.fact(i64 %n) #0 {
entry:
%.71 = icmp eq i64 %n, 0
br i1 %.71, label %entry.endif, label %entry.else.preheader
entry.else.preheader: ; preds = %entry
br label %entry.else
entry.else: ; preds = %entry.else.preheader, %entry.else
%n.tr3 = phi i64 [ %.20, %entry.else ], [ %n, %entry.else.preheader ]
%accumulator.tr2 = phi i64 [ %.22, %entry.else ], [ 1, %entry.else.preheader ]
%.20 = add i64 %n.tr3, -1
%.22 = mul i64 %n.tr3, %accumulator.tr2
%.7 = icmp eq i64 %.20, 0
br i1 %.7, label %entry.endif.loopexit, label %entry.else
entry.endif.loopexit: ; preds = %entry.else
%.22.lcssa = phi i64 [ %.22, %entry.else ]
br label %entry.endif
entry.endif: ; preds = %entry.endif.loopexit, %entry
%accumulator.tr.lcssa = phi i64 [ 1, %entry ], [ %.22.lcssa, %entry.endif.loopexit ]
ret i64 %accumulator.tr.lcssa
}
attributes #0 = { nounwind readnone }
请注意，递归的fact函数已被优化为等效的循环版本。
