gcc不同优化程度生成的代码功能不一样？

源码如下：
#include<stdio.h>
int
main(void) {
int a;
scanf("%d", &a);
if (a < 5 || a > 6)
puts("fail");
else
puts("succeed");
}

分别用不同优化参数编译
gcc a.c -o a.out
gcc a.c -O1 -o b.out

反编译a.out得到的代码是这样的
undefined8 main(void)
{
undefined8 uVar1;
int64_t in_FS_OFFSET;
int32_t var_ch;
int64_t canary;
canary = *(int64_t *)(in_FS_OFFSET + 0x28);
sym.imp.__isoc99_scanf(0x2004, &var_ch);
if ((var_ch < 5) || (6 < var_ch)) {
sym.imp.puts("fail");
} else {
sym.imp.puts("succeed");
}
uVar1 = 0;
if (canary != *(int64_t *)(in_FS_OFFSET + 0x28)) {
uVar1 = sym.imp.__stack_chk_fail();
}
return uVar1;
}

反编译b.out得到的代码是这样的
undefined8 main(void)
{
undefined8 uVar1;
undefined8 extraout_RDX;
int64_t iVar2;
int32_t *piVar3;
uint32_t uVar4;
int64_t in_FS_OFFSET;
int32_t iStack20;
int64_t iStack16;
iStack16 = *(int64_t *)(in_FS_OFFSET + 0x28);
piVar3 = &iStack20;
sym.imp.__isoc99_scanf(0x2004, piVar3);
if (iStack20 - 5U < 2) {
uVar4 = 0x200c;
sym.imp.puts("succeed");
} else {
uVar4 = 0x2007;
sym.imp.puts("fail");
}
if (iStack16 != *(int64_t *)(in_FS_OFFSET + 0x28)) {
sym.imp.__stack_chk_fail();
sym._init();
iVar2 = 0;
do {
uVar1 = (**(code **)(segment.LOAD3 + iVar2 * 8))((uint64_t)uVar4, piVar3, extraout_RDX);
iVar2 = iVar2 + 1;
} while (iVar2 != 1);
return uVar1;
}
return 0;
}

问题在这里
这是gcc a.c -o a.out的代码
if ((var_ch < 5) || (6 < var_ch)) {
sym.imp.puts("fail");
} else {
sym.imp.puts("succeed");
}
这是gcc a.c -O1 -o b.out的代码
if (iStack20 - 5U < 2) {
uVar4 = 0x200c;
sym.imp.puts("succeed");
} else {
uVar4 = 0x2007;
sym.imp.puts("fail");
}
为什么加了优化参数后判断条件少了， 但是我执行的时候效果还是一样的。

谁知道写gcc的大佬怎么设计的

解决了， 详情
https://stackoverflow.com/questions/60280240/strange-decompilation-when-using-gcc-with-different-optimization/60280459#60280459

Here, an integer underflow is exploited. The operation
iStack20 - 5U
is performed unsigned due to the implicit conversion rules when the two operands differ in type (C standard 6.3.1.8(1)):
Otherwise [signedness differs], if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type.
So if iStack20 is < 5, then the result will be very large (somewhere near UINT_MAX) , hence the comparison
iStack20 - 5U < 2
will be false. If iStack20 is > 6, then the result will be greater than 2 anyway. So the program logic is preserved.