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Diffstat (limited to 'gitlab/issues/target_s390x/host_missing/accel_TCG/618.toml')
| -rw-r--r-- | gitlab/issues/target_s390x/host_missing/accel_TCG/618.toml | 103 |
1 files changed, 103 insertions, 0 deletions
diff --git a/gitlab/issues/target_s390x/host_missing/accel_TCG/618.toml b/gitlab/issues/target_s390x/host_missing/accel_TCG/618.toml new file mode 100644 index 00000000..cd397f72 --- /dev/null +++ b/gitlab/issues/target_s390x/host_missing/accel_TCG/618.toml @@ -0,0 +1,103 @@ +id = 618 +title = "overflow condition code determined incorrectly after subtraction on s390x" +state = "closed" +created_at = "2021-09-12T22:42:18.749Z" +closed_at = "2022-04-02T17:38:46.614Z" +labels = ["accel: TCG", "kind::Bug", "target: s390x", "workflow::Confirmed"] +url = "https://gitlab.com/qemu-project/qemu/-/issues/618" +host-os = "Ubuntu 20.04" +host-arch = "x86_64" +qemu-version = "6.1.0" +guest-os = "Linux" +guest-arch = "s390x" +description = """Paul Eggert found this bug, just by taking a look at the file `qemu/target/s390x/tcg/cc_helper.c`. + +The following program +[foo.c](/uploads/c1f425684fd661c4437950d7d8ddf31d/foo.c) +``` +#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)); + } +} +``` +should print +``` +a = 0x0, b = 0x80000000 +no_overflow = 0 +a = 0x0, b = 0x8000000000000000 +no_overflow = 0 +``` +However, when compiled as an s390x program and executed through qemu 6.1.0 (Linux user-mode), it prints 'no_overflow = 1' twice. +``` +$ s390x-linux-gnu-gcc-10 --version +s390x-linux-gnu-gcc-10 (Ubuntu 10.3.0-1ubuntu1~20.04) 10.3.0 +``` + +``` +$ s390x-linux-gnu-gcc-10 -static foo.c +$ ~/inst-qemu/6.1.0/bin/qemu-s390x a.out +a = 0x0, b = 0x80000000 +no_overflow = 1 +a = 0x0, b = 0x8000000000000000 +no_overflow = 1 +``` + +``` +$ s390x-linux-gnu-gcc-10 -O2 -static foo.c +$ ~/inst-qemu/6.1.0/bin/qemu-s390x a.out +a = 0x0, b = 0x80000000 +no_overflow = 1 +a = 0x0, b = 0x8000000000000000 +no_overflow = 1 +``` + +The code generated by 's390x-linux-gnu-gcc-10 -O2' makes use of the 'o' (overflow / ones) condition code: +``` +overflow_64: + lgr %r1,%r2 ;; copy a into %r1 + lghi %r2,0 + sgr %r1,%r3 ;; subtract b from a + bnor %r14 ;; if no overflow, return %r2 = 0 + lghi %r2,1 + br %r14 ;; otherwise, return %r2 = 1 +``` + +The condition code and the overflow bit are defined in the z/Architecture Principles of Operation (POP) http://publibfi.boulder.ibm.com/epubs/pdf/dz9zr011.pdf page 7-5 / 7-6 / 7-388 : "In mathematical terms, signed addition and subtraction produce a fixed-point overflow when the result is outside the range of representation for signed binary integers." + +I conclude that the bug is in QEMU: QEMU does not set the overflow condition code correctly.""" +reproduce = """[foo.static.s390x](/uploads/e4b79b019db590f3a4b13cac41e57ba6/foo.static.s390x) +(the result of "s390x-linux-gnu-gcc-10 -static -O2 foo.c -o foo.static.s390x") + +1. `qemu-s390x foo.static.s390x`""" +additional = """The attached patch fixes it. +[0002-s390x-Fix-determination-of-overflow-condition-code-a.patch](/uploads/8d414f84fe0ed36bf07bd28f5e7836ab/0002-s390x-Fix-determination-of-overflow-condition-code-a.patch)""" |