简单的 MIPS 指令和编译器
编译器（例如 gcc）生成将某些空内存元素加载到寄存器中的指令是否很常见？就像... lw at,0(sp) 其中内存[sp + 0] = 0。这基本上只是将 0 放入 $at ($R1.) 我问是因为我正在查看可执行文件的十六进制转储（可执行文件是 c++ 文件编译的结果）并且我正在手动验证它，如果我从 objdump 状态入口点开始，我会遇到一条执行此操作的指令。我不确定如果这只是一个常见的编译器操作，我是否应该将其视为错误。这似乎是一个将寄存器归零的糟糕方法。 ADDU $at,$0,$0 会更好。或者 SLL $at,$0,$0..
入口点是 400890。最后 jal 的跳转目标是一个空的内存位置（告诉我可能有问题......）请注意，我之前的示例是有目的地仲裁的。
需要明确的是，-32636+gp 是一个空的内存位置。如果您需要证据，我可以立即发布内存内容:)。
00400890 <__start>:
400890: 03e00021 move zero,ra
400894: 04110001 bal 40089c <__start+0xc>
400898: 00000000 nop
40089c: 3c1c0fc0 lui gp,0xfc0
4008a0: 279c7864 addiu gp,gp,30820
4008a4: 039fe021 addu gp,gp,ra
4008a8: 0000f821 move ra,zero
4008ac: 8f848034 lw a0,-32716(gp)
4008b0: 8fa50000 lw a1,0(sp)
4008b4: 27a60004 addiu a2,sp,4
4008b8: 2401fff8 li at,-8
4008bc: 03a1e824 and sp,sp,at
4008c0: 27bdffe0 addiu sp,sp,-32
4008c4: 8f878054 lw a3,-32684(gp)
4008c8: 8f888084 lw t0,-32636(gp)<------ this instruction
4008cc: 00000000 nop
4008d0: afa80010 sw t0,16(sp)
4008d4: afa20014 sw v0,20(sp)
4008d8: afbd0018 sw sp,24(sp)
4008dc: 8f998068 lw t9,-32664(gp)
4008e0: 00000000 nop
4008e4: 0320f809 jalr t9
4008e8: 00000000 nop
Jal 目标是 4010c0。
4010c0: 8f998010 lw t9,-32752(gp)
4010c4: 03e07821 move t7,ra
4010c8: 0320f809 jalr t9
Is it common for compilers (gcc for instance) to generate an instruction that loads some empty memory element into a register? Like... lw at,0(sp) where memory[sp + 0] = 0. This basically just places 0 into $at ($R1.) I ask because I'm looking through an executable file's hex dump (executable file is the result of the compilation of a c++ file) and I'm manually verifying it and if I start at the objdump state entry point I run into an instruction that does this. I'm not sure whether I should take this to be an error if it's just a common compiler action. It seems like a poor way to zero a register. ADDU $at,$0,$0 would be better. Or SLL $at,$0,$0..
The entry point is 400890. The jump target of the jal at the end is an empty memory location (tells me something is probably wrong...) Note that my previous example was purposefully arbitrated.
And just to be clear, -32636+gp is an empty memory location. I can post the memory contents at the point if you'd like proof :).
00400890 <__start>:
400890: 03e00021 move zero,ra
400894: 04110001 bal 40089c <__start+0xc>
400898: 00000000 nop
40089c: 3c1c0fc0 lui gp,0xfc0
4008a0: 279c7864 addiu gp,gp,30820
4008a4: 039fe021 addu gp,gp,ra
4008a8: 0000f821 move ra,zero
4008ac: 8f848034 lw a0,-32716(gp)
4008b0: 8fa50000 lw a1,0(sp)
4008b4: 27a60004 addiu a2,sp,4
4008b8: 2401fff8 li at,-8
4008bc: 03a1e824 and sp,sp,at
4008c0: 27bdffe0 addiu sp,sp,-32
4008c4: 8f878054 lw a3,-32684(gp)
4008c8: 8f888084 lw t0,-32636(gp)<------ this instruction
4008cc: 00000000 nop
4008d0: afa80010 sw t0,16(sp)
4008d4: afa20014 sw v0,20(sp)
4008d8: afbd0018 sw sp,24(sp)
4008dc: 8f998068 lw t9,-32664(gp)
4008e0: 00000000 nop
4008e4: 0320f809 jalr t9
4008e8: 00000000 nop
Jal target is 4010c0.
4010c0: 8f998010 lw t9,-32752(gp)
4010c4: 03e07821 move t7,ra
4010c8: 0320f809 jalr t9
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也许它被放置在跳转语句之后？如果是这样，则该语句在跳转发生之前运行，并且可能是一条不执行任何操作的指令 (nop)。除此之外，它可能只是编译器的优化设置较低。另一种可能性是编译器保留了 CPU 标志字段。移动并添加带有标志的游戏，而我不相信负载会这样做。
Perhaps it's being placed after a jump statement? If so, that statement is run before the jump occurs and could be a do nothing instruction (nop). Beyond that, it could just be the compiler on a lower optimization setting. Another possibility is that the compiler is preserving the CPU flags field. Shift and Add play with flags while a load I don't believe does.
这看起来像 CRT 代码。我认为这段代码正在将操作系统传递的一些参数加载到 $a0 和 $a1 寄存器中。可能会在堆栈上传递一些较大的结构，并且代码正在将该结构加载到正确的堆栈位置。
该代码可能不是由 C 编译器生成的，而是在汇编中手工编码的。
This looks like CRT code. I think this code is loading some parameters passed by the OS in to $a0 and $a1 registers. Probably some larger structure is passed on the stack and code is loading that structure in to correct stack location.
This code is probably not generated by C compiler but hand coded in assembly.