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Description
For the below example, we can observe that the function parameters are not stored in stack slot as per ABI, instead being stored in other area.
The example is available here https://godbolt.org/z/Gfevrzh93
#include <stdio.h>
int __attribute__((noinline)) foo(long* y) {
long **x = &y - 6; // needs to be &y+2 in clang, but as per ABI that is not recorded
printf("return address: %p\n", *(x+2));
return **x;
}
/*
int __attribute__((noinline)) foo(long* y) {
long **x = __builtin_frame_address(1);
printf("return address: %p\n", *(x+2));
return **x;
}
*/
int __attribute__((noinline)) bar() {
long y = 200;
return 1 + foo(&y);
}
int __attribute__((noinline)) xyz() {
return 2 + bar();
}
int main() {
return xyz();
}
The below is the explanation of the discrepancies.
// PowerPC gcc compiler at -O0
.L.foo(long*):
mflr 0
std 0,16(1)
std 31,-8(1)
stdu 1,-144(1)
mr 31,1
std 3,192(31) <--- Right: Stored at parameter area of the caller
addi 9,31,144 <--- Got the frame pointer
std 9,112(31)
ld 9,112(31)
addi 9,9,16
ld 9,0(9) <--- got the return address
mr 4,9
addis 3,2,.LC0@toc@ha
addi 3,3,.LC0@toc@l
bl printf
// PowerPc clang Compiler at -O0
.Lfunc_begin0:
mflr 0
std 31, -8(1)
stdu 1, -144(1)
std 0, 160(1)
mr 31, 1
std 3, 128(31) <--- Wrong: Stored in SAVE reg area instead of caller parameter area
addi 3, 31, 128
addi 3, 3, -48
std 3, 120(31)
ld 3, 120(31)
ld 4, 16(3)
addis 3, 2, .L.str@toc@ha
addi 3, 3, .L.str@toc@l
bl printf