IR simple example and meaning

Source code:
int a, b;
int fn1() {
  char c;
  if (b)
    return 1;
  c = fn1 == a;
  return c;
}
~

Corresponding IR:

; ModuleID = 'utf8.i'
source_filename = "utf8.i"
target datalayout = "e-m:e-i64:64-n32:64"
target triple = "powerpc64le-unknown-linux-gnu"

 ---> Every global variable begins with @
@b = common local_unnamed_addr global i32 0, align 4  
@a = common local_unnamed_addr global i32 0, align 4

---> clang IR use ';' (semi colon) as comments symbol.
; Function Attrs: norecurse nounwind readonly
define signext i32 @fn1() #0 {  ---> Global function fn1
entry:        ---> label entry, which denotes a start of a block.
 
  %0 = load i32, i32* @b, align 4, !tbaa !1   --> every local variable begins with %
 Here the first i32, means the type of load, it loads as i32, the second i32* @b, means
it loads the value from global variable b, with alignment 4 bytes.


  %tobool = icmp eq i32 %0, 0   ---> compare the result i32 %0 (temp variable) from 
last step with 0 and assign the compared bool result to a temp variable named %tobool.
 The above code correspond to the user code if (b)
 
  br i1 %tobool, label %if.end, label %cleanup
---> branch  according to one bit type (i1) variable %tobool value, if true goto %if.end, else goto label%cleanup.

---> The start of if.end block (label name is if.end)
if.end:                                           ; preds = %entry
  %1 = load i32, i32* @a, align 4, !tbaa !1 ---> Load global variable a to a temp variable %1, i32 type, usually this is put on a virtual register, you can consider %1 as r1.

  %conv = sext i32 %1 to i64   ---> sign extend %1 from i32 to i64 and save it to %conv
  %2 = inttoptr i64 %conv to i32 (...)*            ---> convert int to pointer from last step to i32(...) * 
 , i.e., to a function pointer, save the converted result    to %2
 
  %cmp = icmp eq i32 (...)* %2, bitcast (i32 ()* @fn1 to i32 (...)*)  ---> compare equal, function pointer converted from last step: i32 (...)* %2 with bitcast (i32 ()* @fn1 to i32 (...)*), this corresponds to the fn==a in the user source code

  %conv3 = zext i1 %cmp to i32 --> zero extend bit 1 variable %cmp to i32, i.e. convert bool to int
  br label %cleanup ---> branch to label %cleanup to do some clean up work before exit.  Here %conv3 corresponds to variable c

cleanup:                                          ; preds = %entry, %if.end  ---> some preceding labels
  %retval.0 = phi i32 [ %conv3, %if.end ], [ 1, %entry ] 
  ret i32 %retval.0
the return (ret) value is a phi node (a function of the temp variable %conv3 in block %if.end and the constant value 1 in block %entry. This corresponds to return 1; return c
}

程序员修炼之道读书笔记 48

The palest ink is better than the best memory.

好脑子不如烂笔头。

Treat English as Just Another Programming Language

把英语当成另外一种编程语言。 （那就天天编程了，哈哈）

Build Documentation In, Don't Bolt It On

把文档建在里面，不要拴在外面。

clang and llvm: patch, -target, -ppc-asm-full-reg-names

18. How to specify different triple for clang?
-target powerpc64-unknown-unknown

19. How to apply a patch?
patch < ~/patches/test.diff

21. How to dump all registers with name?
clang++ teset.cpp  -S -O2 -o - -mllvm -ppc-asm-full-reg-names

程序员修炼之道读书笔记 47

Use Saboteurs to Test Your Testing

让搞破坏的人测试你的测试用例。

When writing tests, make sure that alarms sound when they should.

好的测试用例应该是出现问题是能够及时捕获。

Test State Coverage, Not Code Coverage

测试状态覆盖，而不是代码覆盖。

If a bug slips through the net of existing tests, you need to add a new test to trap it next time. The Author calls this “tighten the net”.

任何漏网之鱼，都说明你的网有漏洞，你需要修补好你的网。确保不会有同样的鱼从哪里逃走。Bug也是一样的道理。

程序员修炼之道读书笔记 46

过时的文档或信息甚至不如没有文档或信息。

The cobbler's children have no shoes. Often, people who develop software use the poorest tools to do the job.

鞋匠的孩子没鞋穿。软件开发人员通常使用世界上最烂的工具。（此言不谬）

Test Early. Test Often. Test Automatically.

尽早测试，经常测试，自动测试。

Code a little, test a little"

一边编码一边测试，增量开发。当你确认一切正确无误时，批量生产。（这种方法适合于，有些重复性的feature 开发，先让一个work，测试无误，批量生产其他）。这也就是极限编程的持续集成思想。

Coding aren’t done until all the tests run.

编码不算完成，除非所有已有和新增测试通过。