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[01/10] qemu/int128: avoid undefined behavior in int128_lshift

Message ID 20220330202515.66554-2-lucas.araujo@eldorado.org.br
State New
Headers show
Series VDIV/VMOD Implementation | expand

Commit Message

Lucas Mateus Martins Araujo e Castro March 30, 2022, 8:25 p.m. UTC 
From: Matheus Ferst <matheus.ferst@eldorado.org.br>

Avoid the left shift of negative values in int128_lshift by casting
a/a.hi to unsigned.

Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Signed-off-by: Lucas Mateus Castro (alqotel) <lucas.araujo@eldorado.org.br>
---
 include/qemu/int128.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Peter Maydell March 30, 2022, 9:21 p.m. UTC | #1 
On Wed, 30 Mar 2022 at 21:26, Lucas Mateus Castro(alqotel)
<lucas.araujo@eldorado.org.br> wrote:
>
> From: Matheus Ferst <matheus.ferst@eldorado.org.br>
>
> Avoid the left shift of negative values in int128_lshift by casting
> a/a.hi to unsigned.

We compile with -fwrapv, so left shift of negative integers
should always be well-defined for us.

thanks
-- PMM
diff mbox series

Patch

diff --git a/include/qemu/int128.h b/include/qemu/int128.h
index 2c4064256c..2a19558ac6 100644
--- a/include/qemu/int128.h
+++ b/include/qemu/int128.h
@@ -85,7 +85,7 @@  static inline Int128 int128_rshift(Int128 a, int n)
 
 static inline Int128 int128_lshift(Int128 a, int n)
 {
-    return a << n;
+    return (__uint128_t)a << n;
 }
 
 static inline Int128 int128_add(Int128 a, Int128 b)
@@ -305,7 +305,7 @@  static inline Int128 int128_lshift(Int128 a, int n)
     if (n >= 64) {
         return int128_make128(0, l);
     } else if (n > 0) {
-        return int128_make128(l, (a.hi << n) | (a.lo >> (64 - n)));
+        return int128_make128(l, ((uint64_t)a.hi << n) | (a.lo >> (64 - n)));
     }
     return a;
 }