Joseph noticed the -Wabsolute-value warning newly introduced in GCC 9 and included in -Wextra triggers on some unreachable subexpressions in Glibc code that the C front end otherwise makes an effort to avoid diagnosing. The attached trivial patch guards the warning to avoid these spurious instances.
Tested on x86_64-linux. Similarly to other warnings like it, -Wabsolute-value still triggers in unreachable statements but the only way to deal with those instances would be to move all such warnings into the middle-end. Martin
PR c/89425 - -Wabsolute-value warns in dead subexpressions gcc/c/ChangeLog: PR c/89425 * c-parser.c (sizeof_ptr_memacc_comptypes): Avoid warning in unreachable subexpressions. gcc/testsuite/ChangeLog: PR c/89425 * gcc.dg/Wabsolute-value.c: New test. Index: gcc/c/c-parser.c =================================================================== --- gcc/c/c-parser.c (revision 269041) +++ gcc/c/c-parser.c (working copy) @@ -9374,6 +9374,10 @@ sizeof_ptr_memacc_comptypes (tree type1, tree type static void warn_for_abs (location_t loc, tree fndecl, tree arg) { + /* Avoid warning in unreachable subexpressions. */ + if (c_inhibit_evaluation_warnings) + return; + tree atype = TREE_TYPE (arg); /* Casts from pointers (and thus arrays and fndecls) will generate Index: gcc/testsuite/gcc.dg/Wabsolute-value.c =================================================================== --- gcc/testsuite/gcc.dg/Wabsolute-value.c (nonexistent) +++ gcc/testsuite/gcc.dg/Wabsolute-value.c (working copy) @@ -0,0 +1,57 @@ +/* PR c/89425 - -Wabsolute-value warns in dead subexpressions + { dg-do compile } + { dg-options "-Wabsolute-value -ftrack-macro-expansion=0" } */ + +struct Vals +{ + signed char sc; + signed short ss; + signed int si; + signed long sl; + signed long long sll; + + unsigned char uc; + unsigned short us; + unsigned int ui; + unsigned long ul; + unsigned long long ull; + + float f; + double d; + long double ld; +}; + +#define abs(x) __builtin_abs (x) +#define labs(x) __builtin_labs (x) +#define llabs(x) __builtin_llabs (x) + +#define fabsf(x) __builtin_fabsf (x) +#define fabs(x) __builtin_fabs (x) + + +void tst_warn (struct Vals *p) +{ + /* Verify that "-Wabsolute-value is issued for subexpressions + that are evaluated. */ + + p->uc = 0 ? abs (p->sc) : abs (p->uc); /* { dg-warning "\\\[-Wabsolute-value]" } */ + p->us = 0 ? abs (p->ss) : abs (p->us); /* { dg-warning "\\\[-Wabsolute-value]" } */ + p->ui = 0 ? abs (p->si) : abs (p->ui); /* { dg-warning "\\\[-Wabsolute-value]" } */ + p->ul = 0 ? labs (p->sl) : labs (p->ul); /* { dg-warning "\\\[-Wabsolute-value]" } */ + p->ull = 0 ? llabs (p->sll) : llabs (p->ull); /* { dg-warning "\\\[-Wabsolute-value]" } */ + + p->d = 0 ? fabs (p->d) : fabsf (p->d); /* { dg-warning "\\\[-Wabsolute-value]" } */ +} + +void tst_no_warn (struct Vals *p) +{ + /* Verify that "-Wabsolute-value is not issued for subexpressions + that are not evaluated. */ + + p->uc = 0 ? abs (p->uc) : abs (p->sc); + p->us = 0 ? abs (p->us) : abs (p->ss); + p->ui = 0 ? abs (p->ui) : abs (p->si); + p->ul = 0 ? labs (p->ul) : labs (p->sl); + p->ull = 0 ? llabs (p->ull) : llabs (p->sll); + p->d = 0 ? fabsf (p->d) : fabs (p->d); +}