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authorJulius Werner <jwerner@chromium.org>2020-06-26 23:27:26 -0700
committerPatrick Georgi <pgeorgi@google.com>2020-07-06 06:14:57 +0000
commit63358eaff6058e2fa0ade9b452a7ffd7ce3f4ba8 (patch)
tree6895cd1c0e8eb324771437d2a8ddbc3c5a5bb5c2 /payloads/libpayload
parenta94136f082844b811b4cae25887740edf41503be (diff)
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/libpayload')
-rw-r--r--payloads/libpayload/drivers/video/graphics.c10
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,