https://bugs.kde.org/show_bug.cgi?id=402519
Mark Wielaard <m...@klomp.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|REPORTED |ASSIGNED
Ever confirmed|0 |1
--- Comment #1 from Mark Wielaard <m...@klomp.org> ---
Some more details from private email(s).
This was originally reported as an issue with GMP on power9.
Here is some example code:
File t.c:
#include <stdio.h>
unsigned long f();
int
main ()
{
unsigned long foo = f();
for (int i = 56; i >= 0; i -= 8)
printf ("%c", (char) (foo >> i));
printf ("\n");
return 0;
}
File f.s:
.globl f
.type f, @function
f: li 5, 0x6E63
addis 5, 5, 0x7269
li 6, 0x6C67
addis 6, 6, 0x5661
insrdi 5, 6, 32, 0
li 3, -1
subfo 0, 0, 0 # OV <- 0
addex 4, 3, 3, 0 # OV <- 1
addex 3, 3, 3, 0 # r3 <- 0xff..ff + 0xff..ff + 1 = 0xff..fe = -2
sub 3, 5, 3
blr
$ gcc -std=c99 t.c f.s
$ valgrind ./a.out
Correct output is "Valgrind".
Output under Valgrind: "Valgrine".
There is actually a testcase for addex in valgrind
none/tests/ppc64/test_isa_3_0.c. But it is commented out.
And indeed, it fails when enabled.
The issue seems to be that the XER OV flag is set wrongly.
The code in calculate_XER_OV_64 () does:
/* (argL^argR^-1) & (argL^res) & (1<<63) ? 1:0 */
// i.e. ((both_same_sign) & (sign_changed) & (sign_mask))
Which checks for two's complements overflow.
But addex is akin to adde except that addex inputs carry from OV and outputs
carry to OV, while adde of course uses CA.
The meaning of OV for other instructions is signed overflow.
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