默认情况下,用于Risc-V的GCC会在nop
指令之后生成call
指令:
$ cat test.c
void g();
void f() {
g();
}
$ riscv64-unknown-elf-gcc -S test.c -o -
[...]
f:
addi sp,sp,-16
sd ra,8(sp)
sd s0,0(sp)
addi s0,sp,16
call g
nop #### <-----------here
ld ra,8(sp)
ld s0,0(sp)
addi sp,sp,16
jr ra
.size f, .-f
.ident "GCC: (GNU) 8.3.0"
我希望在针对具有branch delay slot的体系结构时,但我的理解是Risc-V不是这样的体系结构。实际上,当使用
nop
或更高版本进行编译时,-O1
消失了。只是GCC中的“错误”会发出
nop
作为具有延迟插槽的体系结构的剩余部分,还是出现此nop
指令的实际原因? 最佳答案
这不是一个完整的答案,但至少是试图弄清点头出现的原因。我坚信这是具有延迟插槽的体系结构中的错误/遗留问题(因为它是在第一个RTL传递中添加-扩展)。
在调查中,GCC有两种类型的通行证:Tree和RTL。要查看它们的实际效果,请准备2个文件夹,因为它将有许多文件noopt
和opt
,并使用-fdump-tree-all-raw -fdump-rtl-all
查看中间结果。 Tree阶段的最后阶段给出了(noopt情况):
$ cat noopt/test.c.232t.optimized
;; Function f (f, funcdef_no=0, decl_uid=1549, cgraph_uid=0, symbol_order=0)
f ()
{
<bb 2> :
gimple_call <g, NULL>
gimple_return <NULL NULL>
}
opt大小写(
-O1
)的差异可以忽略不计:$ diff -u noopt/test.c.232t.optimized opt/test.c.232t.optimized
--- noopt/test.c.232t.optimized 2019-09-03 14:48:02.874071927 +0200
+++ opt/test.c.232t.optimized 2019-09-03 14:48:29.550278667 +0200
@@ -3,7 +3,7 @@
f ()
{
- <bb 2> :
+ <bb 2> [local count: 1073741825]:
gimple_call <g, NULL>
gimple_return <NULL NULL>
RTL通过(扩展)的第一阶段是不同的:
$ cat noopt/test.c.234r.expand
;; Function f (f, funcdef_no=0, decl_uid=1549, cgraph_uid=0, symbol_order=0)
;; Generating RTL for gimple basic block 2
try_optimize_cfg iteration 1
Merging block 3 into block 2...
Merged blocks 2 and 3.
Merged 2 and 3 without moving.
Merging block 4 into block 2...
Merged blocks 2 and 4.
Merged 2 and 4 without moving.
try_optimize_cfg iteration 2
;;
;; Full RTL generated for this function:
;;
(note 1 0 3 NOTE_INSN_DELETED)
(note 3 1 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)
(note 2 3 5 2 NOTE_INSN_FUNCTION_BEG)
(call_insn 5 2 8 2 (parallel [
(call (mem:SI (symbol_ref:DI ("g") [flags 0x41] <function_decl 0x7fbc2827a400 g>) [0 g S4 A32])
(const_int 0 [0]))
(clobber (reg:SI 1 ra))
]) "../test.c":3 -1
(nil)
(nil))
(insn 8 5 0 2 (const_int 0 [0]) "../test.c":4 -1
(nil))
-O1
的不同之处在于只是删除了const_int 0 [0]
,最终将导致nop
:$ diff -u noopt/test.c.234r.expand opt/test.c.234r.expand
--- noopt/test.c.234r.expand 2019-09-03 14:48:02.874071927 +0200
+++ opt/test.c.234r.expand 2019-09-03 14:48:29.550278667 +0200
@@ -25,12 +25,10 @@
(note 1 0 3 NOTE_INSN_DELETED)
(note 3 1 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)
(note 2 3 5 2 NOTE_INSN_FUNCTION_BEG)
-(call_insn 5 2 8 2 (parallel [
- (call (mem:SI (symbol_ref:DI ("g") [flags 0x41] <function_decl 0x7fbc2827a400 g>) [0 g S4 A32])
+(call_insn 5 2 0 2 (parallel [
+ (call (mem:SI (symbol_ref:DI ("g") [flags 0x41] <function_decl 0x7f0bdec1f400 g>) [0 g S4 A32])
(const_int 0 [0]))
(clobber (reg:SI 1 ra))
]) "../test.c":3 -1
(nil)
(nil))
-(insn 8 5 0 2 (const_int 0 [0]) "../test.c":4 -1
- (nil))
关于gcc - 为什么调用后GCC for Risc-V会产生nop指令,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/57695031/