46 lines
1.9 KiB
ReStructuredText
46 lines
1.9 KiB
ReStructuredText
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.. title:: clang-tidy - performance-no-int-to-ptr
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performance-no-int-to-ptr
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=========================
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Diagnoses every integer to pointer cast.
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While casting an (integral) pointer to an integer is obvious - you just get
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the integral value of the pointer, casting an integer to an (integral) pointer
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is deceivingly different. While you will get a pointer with that integral value,
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if you got that integral value via a pointer-to-integer cast originally,
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the new pointer will lack the provenance information from the original pointer.
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So while (integral) pointer to integer casts are effectively no-ops,
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and are transparent to the optimizer, integer to (integral) pointer casts
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are *NOT* transparent, and may conceal information from optimizer.
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While that may be the intention, it is not always so. For example,
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let's take a look at a routine to align the pointer up to the multiple of 16:
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The obvious, naive implementation for that is:
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.. code-block:: c++
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char* src(char* maybe_underbiased_ptr) {
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uintptr_t maybe_underbiased_intptr = (uintptr_t)maybe_underbiased_ptr;
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uintptr_t aligned_biased_intptr = maybe_underbiased_intptr + 15;
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uintptr_t aligned_intptr = aligned_biased_intptr & (~15);
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return (char*)aligned_intptr; // warning: avoid integer to pointer casts [performance-no-int-to-ptr]
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}
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The check will rightfully diagnose that cast.
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But when provenance concealment is not the goal of the code, but an accident,
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this example can be rewritten as follows, without using integer to pointer cast:
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.. code-block:: c++
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char*
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tgt(char* maybe_underbiased_ptr) {
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uintptr_t maybe_underbiased_intptr = (uintptr_t)maybe_underbiased_ptr;
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uintptr_t aligned_biased_intptr = maybe_underbiased_intptr + 15;
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uintptr_t aligned_intptr = aligned_biased_intptr & (~15);
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uintptr_t bias = aligned_intptr - maybe_underbiased_intptr;
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return maybe_underbiased_ptr + bias;
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}
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