UBSan should not warn on integer overflow with downcast or bitwise-AND mask

[Explorer09]

Test code

#include <stdint.h>
#include <stdio.h>

static inline unsigned int popCount8(uint8_t x) {
   // Formula borrowed from https://github.com/llvm/llvm-project/issues/79823
   uint32_t n = (uint32_t)(x * 0x08040201U);
   n = ((((n >> 3) & 0x11111111U) * 0x11111111U) & 0xF0000000U) >> 28;
   return n;
}

int main(void) {
   volatile uint8_t x;
   x = 0xFF;
   // Expected output: 8
   printf("%u\n", popCount8(x));
   return 0;
}

Compile with: clang -fsanitize=integer -o test test.c
Then run with ./test

Actual behavior

test1.c:6:30: runtime error: unsigned integer overflow: 255 * 134480385 cannot be represented in type 'unsigned int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior test1.c:6:30 in 
test1.c:7:35: runtime error: unsigned integer overflow: 286331153 * 286331153 cannot be represented in type 'unsigned int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior test1.c:7:44 in 

Expected behavior

No errors and no warnings.

The compiler and UndefinedBehaviorSanitizer should not warn about both overflow cases as I have made downcasts and bitwise-AND masks to tell that the "overflows" are intentional.

• (uint32_t)(x * 0x08040201U)
• ((x * 0x11111111U) & 0xF0000000U)

While I can workaround the bug by adding a upcast like this...

• (uint32_t)((uint64_t)x * 0x08040201U)
• (((uint64_t)x * 0x11111111U) & 0xF0000000U)

... I would rather like conciseness and not needing such casts.