Clang的使用

Clang 版本

$ clang --version clang version 7.0.1-12 (tags/RELEASE_701/final) Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin

使用Clang的例子

编写一个简单的c代码

// t.c

typedef float V __attribute__((vector_size(16)));

V foo(V a, V b) { return a+b*a; }

__attribute__ 用于属性声明。 vector_size(16) 声明V的大小为16字节

预处理

$ clang t.c -E

# 1 "t.c"

# 1 "" 1

# 1 "" 3

# 349 "" 3

# 1 "" 1

# 1 "" 2

# 1 "t.c" 2

typedef float V __attribute__((vector_size(16)));

V foo(V a, V b) { return a+b*a;}

类型检查

$ clang -fsyntax-only t.c 这里什么都没有输出

GCC 选项

clang -fsyntax-only t.c -pedantic 这里也什么都没有输出。就很奇怪，和clang官方文档不符合.

完美打印抽象语法树

$ clang -cc1 t.c -ast-print typedef attribute((vector_size(4 * sizeof(float)))) float V; V foo(V a, V b) { return a + b * a; } 请注意，-cc1 参数表示应该运行编译器前端，而不是驱动程序。 编译器前端有几个额外的 Clang 特定功能，这些功能未通过 GCC 兼容驱动程序接口公开。

使用LLVM代码生成

$ clang t.c -S -emit-llvm -o -

; ModuleID = 't.c'

source_filename = "t.c"

target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

target triple = "x86_64-pc-linux-gnu"

; Function Attrs: noinline nounwind optnone uwtable

define dso_local <4 x float> @foo(<4 x float>, <4 x float>) #0 {

%3 = alloca <4 x float>, align 16

%4 = alloca <4 x float>, align 16

store <4 x float> %0, <4 x float>* %3, align 16

store <4 x float> %1, <4 x float>* %4, align 16

%5 = load <4 x float>, <4 x float>* %3, align 16

%6 = load <4 x float>, <4 x float>* %4, align 16

%7 = load <4 x float>, <4 x float>* %3, align 16

%8 = fmul <4 x float> %6, %7

%9 = fadd <4 x float> %5, %8

ret <4 x float> %9

}

attributes #0 = { noinline nounwind optnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0}

!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}

!1 = !{!"clang version 7.0.1-12 (tags/RELEASE_701/final)"}

clang -fomit-frame-pointer -O3 -S -o - t.c # On x86_64

.text

.file "t.c"

.globl foo # -- Begin function foo

.p2align 4, 0x90

.type foo,@function

foo: # @foo

.cfi_startproc

# %bb.0:

mulps %xmm0, %xmm1

addps %xmm1, %xmm0

retq

.Lfunc_end0:

.size foo, .Lfunc_end0-foo

.cfi_endproc

# -- End function

.ident "clang version 7.0.1-12 (tags/RELEASE_701/final)"

.section ".note.GNU-stack","",@progbits

.addrsig

参考文献

Getting Started: Building and Running Clang