mirror of https://github.com/acidanthera/audk.git
75505d1611
If we have to negate UnsignedResult (due to exactly one of Multiplicand
and Multiplier being negative), and UnsignedResult is exactly
MIN_INT64_MAGNITUDE (value 2^63), then the statement
*Result = - ((INT64)UnsignedResult);
invokes both implementation-defined behavior and undefined behavior.
First, MIN_INT64_MAGNITUDE is not representable as INT64, therefore the
result of the (inner) conversion
(INT64)MIN_INT64_MAGNITUDE
is implementation-defined, or an implementation-defined signal is raised,
according to ISO C99 6.3.1.3p3.
Second, if we assume that the C language implementation defines the
conversion to INT64 simply as reinterpreting the bit pattern
0x8000_0000_0000_0000 as a signed integer in two's complement
representation, then the conversion immediately produces the negative
value MIN_INT64 (value -(2^63)). In turn, the (outer) negation
-(MIN_INT64)
invokes undefined behavior, because the mathematical result of the
negation, namely 2^63, cannot be represented in an INT64 object. (Not even
mentioning the fact that the mathematical result would be incorrect.) In
practice, the undefined negation of MIN_INT64 happens to produce an
unchanged, valid-looking result on x86, i.e. (-(MIN_INT64)) == MIN_INT64.
We can summarize this as the undefined -- effectless -- negation canceling
out the botched -- auto-negating -- implementation-defined conversion.
Instead of relying on such behavior, dedicate a branch to this situation:
assign MIN_INT64 directly. The branch can be triggered e.g. by multiplying
(2^62) by (-2).
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Michael D Kinney <michael.d.kinney@intel.com>
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| .. | ||
| Include | ||
| Library | ||
| MdePkg.dec | ||
| MdePkg.dsc | ||
| MdePkg.uni | ||
| MdePkgExtra.uni | ||