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* [gcc r11-6823] c++: Avoid UB in signed shift [PR 98625]
@ 2021-01-20 19:42 Nathan Sidwell
0 siblings, 0 replies; only message in thread
From: Nathan Sidwell @ 2021-01-20 19:42 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:911f797a9be2dc8ef59f5d5bd6d68baf650b8822
commit r11-6823-g911f797a9be2dc8ef59f5d5bd6d68baf650b8822
Author: Nathan Sidwell <nathan@acm.org>
Date: Wed Jan 20 09:21:02 2021 -0800
c++: Avoid UB in signed shift [PR 98625]
I'd forgotten that left shifting a negative value is UB until C++20.
Insert some casts to do unsigned shifts.
PT c++/98625
gcc/cp/
* module.cc (bytes_in::i, bytes_in::wi): Avoid left shift of
signed type.
Diff:
---
gcc/cp/module.cc | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 3b40c7ead05..7f88e3fea89 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -878,9 +878,12 @@ bytes_in::i ()
v &= 0xf;
if (v & 0x8)
v |= -1 ^ 0x7;
+ /* unsigned necessary due to left shifts of -ve values. */
+ unsigned uv = unsigned (v);
if ((ptr = read (++bytes)))
while (bytes--)
- v = (v << 8) | (*ptr++ & 0xff);
+ uv = (uv << 8) | (*ptr++ & 0xff);
+ v = int (uv);
}
else if (v & 0x40)
v |= -1 ^ 0x3f;
@@ -969,9 +972,12 @@ bytes_in::wi ()
v &= 0xf;
if (v & 0x8)
v |= -1 ^ 0x7;
+ /* unsigned necessary due to left shifts of -ve values. */
+ unsigned HOST_WIDE_INT uv = (unsigned HOST_WIDE_INT) v;
if ((ptr = read (++bytes)))
while (bytes--)
- v = (v << 8) | (*ptr++ & 0xff);
+ uv = (uv << 8) | (*ptr++ & 0xff);
+ v = (HOST_WIDE_INT) uv;
}
else if (v & 0x40)
v |= -1 ^ 0x3f;
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