http://llvm.org/bugs/show_bug.cgi?id=12290
Bug #: 12290
Summary: Overflow builtins
Product: clang
Version: unspecified
Platform: All
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P
Component: Frontend
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Classification: Unclassified
It would be great if Clang provides builtins to help C/C++ programmers perform
overflow checking. Here's an possible usage, assuming __umuloz(a, b, p) stores
the result of size_t multiplication a * b to p and returns 0 if no overflow
happens.
void *malloc_array(size_t n, size_t size)
{
size_t bytes;
if (__umuloz(n, size, &bytes))
return NULL;
return malloc(bytes);
}
Other overflow builtins could include __[s|u][add|sub|mul][16|32|64].
These builtins are convenient to use and less error-prone, compared to popular
ad-hoc checks such as (size && n > SIZE_MAX / size).
LLVM (esp. instcombine) doesn't recognize such ad-hoc checking patterns,
either, and generates code using div rather than a simple jno. These builtins
could be easily lowered to the LLVM overflow intrinsics llvm.*.with.overflow.*
for better code generation.
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