tidy - bugprone-implicit-widening-of-multiplication-result — Extra Clang Tools 22.0.0git documentation (original) (raw)

The check diagnoses instances where a result of a multiplication is implicitly widened, and suggests (with fix-it) to either silence the code by making widening explicit, or to perform the multiplication in a wider type, to avoid the widening afterwards.

This is mainly useful when operating on very large buffers. For example, consider:

void zeroinit(char* base, unsigned width, unsigned height) { for(unsigned row = 0; row != height; ++row) { for(unsigned col = 0; col != width; ++col) { char* ptr = base + row * width + col; *ptr = 0; } } }

This is fine in general, but if width * height overflows, you end up wrapping back to the beginning of baseinstead of processing the entire requested buffer.

Indeed, this only matters for pretty large buffers (4GB+), but that can happen very easily for example in image processing, where for that to happen you “only” need a ~269MPix image.

Options¶

UseCXXStaticCastsInCppSources¶

When suggesting fix-its for C++ code, should C++-style static_cast<>()’s be suggested, or C-style casts. Defaults to true.

When suggesting to include the appropriate header in C++ code, should <cstddef> header be suggested, or <stddef.h>. Defaults to true.

IgnoreConstantIntExpr¶

If the multiplication operands are compile-time constants (like literals or are constexpr) and fit within the source expression type, do not emit a diagnostic or suggested fix. Only considers expressions where the source expression is a signed integer type. Defaults to false.

Examples:

long mul(int a, int b) { return a * b; // warning: performing an implicit widening conversion to type 'long' of a multiplication performed in type 'int' }

char* ptr_add(char *base, int a, int b) { return base + a * b; // warning: result of multiplication in type 'int' is used as a pointer offset after an implicit widening conversion to type 'ssize_t' }

char ptr_subscript(char *base, int a, int b) { return base[a * b]; // warning: result of multiplication in type 'int' is used as a pointer offset after an implicit widening conversion to type 'ssize_t' }