diff options
| author | Julius Werner <jwerner@chromium.org> | 2020-06-26 23:27:26 -0700 |
|---|---|---|
| committer | Patrick Georgi <pgeorgi@google.com> | 2020-07-06 06:14:57 +0000 |
| commit | 63358eaff6058e2fa0ade9b452a7ffd7ce3f4ba8 (patch) | |
| tree | 6895cd1c0e8eb324771437d2a8ddbc3c5a5bb5c2 /payloads | |
| parent | a94136f082844b811b4cae25887740edf41503be (diff) | |
| download | coreboot-63358eaff6058e2fa0ade9b452a7ffd7ce3f4ba8.tar.xz | |
libpayload: cbgfx: Fix add_fractions() overflow reduction
log2(1) is 0 and log2(0) is -1. If we have the int64_t 0xffffffff then
log2(0xffffffff >> 31) = log2(0x1) = 0, so the current reduction code
would not shift. That's a bad idea, though, since 0xffffffff when
interpreted as an int32_t would become a negative number.
We need to always shift one more than the current code does to get a
safe reduction. This also means we can get rid of another compare/branch
since -1 is the smallest result log2() can return, so the shift can no
longer go negative now.
Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: Ib1eb6364c35c26924804261c02171139cdbd1034
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42845
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-by: Joel Kitching <kitching@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Diffstat (limited to 'payloads')
| -rw-r--r-- | payloads/libpayload/drivers/video/graphics.c | 10 |
1 files changed, 3 insertions, 7 deletions
diff --git a/payloads/libpayload/drivers/video/graphics.c b/payloads/libpayload/drivers/video/graphics.c index 13eac28ea5..fa72c9b743 100644 --- a/payloads/libpayload/drivers/video/graphics.c +++ b/payloads/libpayload/drivers/video/graphics.c @@ -85,13 +85,9 @@ static void add_fractions(struct fraction *out, n = (int64_t)f1->n * f2->d + (int64_t)f2->n * f1->d; d = (int64_t)f1->d * f2->d; /* Simplest way to reduce the fraction until fitting in int32_t */ - shift = log2(MAX(ABS(n), ABS(d)) >> 31); - if (shift > 0) { - n >>= shift; - d >>= shift; - } - out->n = n; - out->d = d; + shift = log2(MAX(ABS(n), ABS(d)) >> 31) + 1; + out->n = n >> shift; + out->d = d >> shift; } static void add_scales(struct scale *out, |