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Re: [PATCH 2/2] target/s390x: Fix determination of overflow condition co
From: |
David Hildenbrand |
Subject: |
Re: [PATCH 2/2] target/s390x: Fix determination of overflow condition code after subtraction |
Date: |
Wed, 30 Mar 2022 10:57:15 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 |
On 23.03.22 17:26, Thomas Huth wrote:
> Reported by Paul Eggert in
> https://lists.gnu.org/archive/html/bug-gnulib/2021-09/msg00050.html
>
> This program currently prints different results when run with TCG instead
> of running on real s390x hardware:
>
> #include <stdio.h>
>
> int overflow_32 (int x, int y)
> {
> int sum;
> return __builtin_sub_overflow (x, y, &sum);
> }
>
> int overflow_64 (long long x, long long y)
> {
> long sum;
> return __builtin_sub_overflow (x, y, &sum);
> }
>
> int a1 = 0;
> int b1 = -2147483648;
> long long a2 = 0L;
> long long b2 = -9223372036854775808L;
>
> int main ()
> {
> {
> int a = a1;
> int b = b1;
> printf ("a = 0x%x, b = 0x%x\n", a, b);
> printf ("no_overflow = %d\n", ! overflow_32 (a, b));
> }
> {
> long long a = a2;
> long long b = b2;
> printf ("a = 0x%llx, b = 0x%llx\n", a, b);
> printf ("no_overflow = %d\n", ! overflow_64 (a, b));
> }
> }
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/618
> Suggested-by: Bruno Haible <bruno@clisp.org>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
> target/s390x/tcg/cc_helper.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/target/s390x/tcg/cc_helper.c b/target/s390x/tcg/cc_helper.c
> index e11cdb745d..b2e8d3d9f5 100644
> --- a/target/s390x/tcg/cc_helper.c
> +++ b/target/s390x/tcg/cc_helper.c
> @@ -151,7 +151,7 @@ static uint32_t cc_calc_add_64(int64_t a1, int64_t a2,
> int64_t ar)
>
> static uint32_t cc_calc_sub_64(int64_t a1, int64_t a2, int64_t ar)
> {
> - if ((a1 > 0 && a2 < 0 && ar < 0) || (a1 < 0 && a2 > 0 && ar > 0)) {
> + if ((a1 >= 0 && a2 < 0 && ar < 0) || (a1 < 0 && a2 > 0 && ar > 0)) {
> return 3; /* overflow */
> } else {
> if (ar < 0) {
> @@ -211,7 +211,7 @@ static uint32_t cc_calc_add_32(int32_t a1, int32_t a2,
> int32_t ar)
>
> static uint32_t cc_calc_sub_32(int32_t a1, int32_t a2, int32_t ar)
> {
> - if ((a1 > 0 && a2 < 0 && ar < 0) || (a1 < 0 && a2 > 0 && ar > 0)) {
> + if ((a1 >= 0 && a2 < 0 && ar < 0) || (a1 < 0 && a2 > 0 && ar > 0)) {
> return 3; /* overflow */
> } else {
> if (ar < 0) {
Again, intuitively I'd check for
a) Subtracting a negative number from a positive one -> Adding two
positive numbers should result in the result being bigger than the first
parameter.
a1 > 0 && a2 < 0 && ar < a1
a) Subtracting a positive number from a negative one -> Adding two
negative numbers should result in something that's smaller than the
first parameter
a1 < 0 && a2 > 0 && ar > a1
But maybe I am missing something :)
--
Thanks,
David / dhildenb
- [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, (continued)
- [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, Thomas Huth, 2022/03/23
- Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, David Hildenbrand, 2022/03/30
- Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, Thomas Huth, 2022/03/30
- Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, David Hildenbrand, 2022/03/30
- Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, Thomas Huth, 2022/03/30
- Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, David Hildenbrand, 2022/03/30
- Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, Thomas Huth, 2022/03/30
- Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, Thomas Huth, 2022/03/30
- Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition, Richard Henderson, 2022/03/30
[PATCH 2/2] target/s390x: Fix determination of overflow condition code after subtraction, Thomas Huth, 2022/03/23
- Re: [PATCH 2/2] target/s390x: Fix determination of overflow condition code after subtraction,
David Hildenbrand <=