UB in negate in mulh/mulhsu

nibrunieAtSi5 · nibrunie · commit fd077496cdf2 · 2025-01-18T09:40:03.000-08:00
Ensuring No negation on -2^63(int64_t) is performed during mulh/mulhsu

Signed-off-by: Nicolas Brunie &lt;82109999+nibrunieAtSi5@users.noreply.github.com&gt;

Update riscv/arith.h

Signed-off-by: Nicolas Brunie &lt;82109999+nibrunieAtSi5@users.noreply.github.com&gt;
diff --git a/riscv/arith.h b/riscv/arith.h
@@ -32,15 +32,15 @@ inline uint64_t mulhu(uint64_t a, uint64_t b)
 inline int64_t mulh(int64_t a, int64_t b)
 {
   int negate = (a < 0) != (b < 0);
-  uint64_t res = mulhu(a < 0 ? -a : a, b < 0 ? -b : b);
+  uint64_t res = mulhu(a < 0 ? -(uint64_t)a : a, b < 0 ? -(uint64_t)b : b);
   return negate ? ~res + ((uint64_t)a * (uint64_t)b == 0) : res;
 }
 
 inline int64_t mulhsu(int64_t a, uint64_t b)
 {
   int negate = a < 0;
-  uint64_t res = mulhu(a < 0 ? -a : a, b);
-  return negate ? ~res + (a * b == 0) : res;
+  uint64_t res = mulhu(a < 0 ? -(uint64_t)a : a, b);
+  return negate ? ~res + ((uint64_t)a * b == 0) : res;
 }
 
 //ref:  https://locklessinc.com/articles/sat_arithmetic/

Original file line number	Diff line number	Diff line change
`@@ -32,15 +32,15 @@ inline uint64_t mulhu(uint64_t a, uint64_t b)`
`32`	`32`	`inline int64_t mulh(int64_t a, int64_t b)`
`33`	`33`	`{`
`34`	`34`	`int negate = (a < 0) != (b < 0);`
`35`		`- uint64_t res = mulhu(a < 0 ? -a : a, b < 0 ? -b : b);`
	`35`	`+ uint64_t res = mulhu(a < 0 ? -(uint64_t)a : a, b < 0 ? -(uint64_t)b : b);`
`36`	`36`	`return negate ? ~res + ((uint64_t)a * (uint64_t)b == 0) : res;`
`37`	`37`	`}`
`38`	`38`
`39`	`39`	`inline int64_t mulhsu(int64_t a, uint64_t b)`
`40`	`40`	`{`
`41`	`41`	`int negate = a < 0;`
`42`		`- uint64_t res = mulhu(a < 0 ? -a : a, b);`
`43`		`- return negate ? ~res + (a * b == 0) : res;`
	`42`	`+ uint64_t res = mulhu(a < 0 ? -(uint64_t)a : a, b);`
	`43`	`+ return negate ? ~res + ((uint64_t)a * b == 0) : res;`
`44`	`44`	`}`
`45`	`45`
`46`	`46`	`//ref: https://locklessinc.com/articles/sat_arithmetic/`