================
----------------
zibi2 wrote:
Here is a concrete cross-compiler test run on z/OS (clang caller, xlc callee):
```
PASS int 2 R2D = 0x0000000000000002
PASS int -1 R2D = 0x00000000ffffffff
PASS int 0x7fffffff R2D = 0x000000007fffffff
PASS int 0 R2D = 0x0000000000000000
PASS int -2147483648 R2D = 0x0000000080000000
```
poison() pre-loads 0xDEADBEEF into the upper 32 bits of R1–R3 before each call.
If clang wrote only R2L, the result for int -1 would be 0xDEADBEEFffffffff. It
is 0x00000000ffffffff so clang zero-extends into the full 64-bit register.
<details>
<summary>caller.c (clang -m64)</summary>
```
/* passes known int values; clang is the thing under test */
extern void witness_r1(unsigned long long *out, int val);
extern void poison(void);
#define ZX(n) ((unsigned long long)(unsigned int)(n))
static void check(const char *label, int val, unsigned long long expected)
{
unsigned long long got = 0;
poison();
witness_r1(&got, val);
if (got == expected)
printf("PASS %-26s R2D = 0x%016llx\n", label, got);
else {
printf("FAIL %-26s R2D = 0x%016llx expected 0x%016llx\n",
label, got, expected);
failures++;
}
}
int main(void)
{
check("int 2", 2, ZX(2));
check("int -1", -1, ZX(-1));
check("int 0x7fffffff", 0x7fffffff, ZX(0x7fffffff));
check("int 0", 0, ZX(0));
check("int -2147483648", -2147483648, ZX(-2147483648));
printf("\n%s\n", failures ? "FAILED" : "ALL PASSED");
return failures != 0;
}
```
</details><details>
<summary>witness.c (xlc -q64) — captures raw R2D before any prologue</summary>
```
/* parameter is unsigned long long, NOT int -- suppresses xlc's LGFR prologue */
void witness_r1(unsigned long long *out, unsigned long long val) {
__asm(" stg 2,0(1)"); /* store raw R2D into *out */
}
```
</details> <details>
<summary>poison.c (xlc -q64) — pre-contaminates upper half</summary>
```
void poison(void) {
__asm(" llihf 1,X'DEADBEEF'");
__asm(" llihf 2,X'DEADBEEF'");
__asm(" llihf 3,X'DEADBEEF'");
}
```
</details>
Without last commit the last sub-test would fail as follows:
`FAIL int -2147483648 R2D = 0xffffffff80000000 expected
0x0000000080000000`
https://github.com/llvm/llvm-project/pull/206833
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