[glibc] Avoid maybe-uninitialized warning in __kernel_rem_pio2

[Andreas Schwab <schwab@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=5aa1ddfcb3374425b7fe9a1389b98a45a47e4a77

commit 5aa1ddfcb3374425b7fe9a1389b98a45a47e4a77
Author: Andreas Schwab <schwab@suse.de>
Date:   Sun Oct 8 18:23:30 2023 +0200

    Avoid maybe-uninitialized warning in __kernel_rem_pio2
    
    With GCC 14 on 32-bit x86 the compiler emits a maybe-uninitialized
    warning:
    
    ../sysdeps/ieee754/dbl-64/k_rem_pio2.c: In function '__kernel_rem_pio2':
    ../sysdeps/ieee754/dbl-64/k_rem_pio2.c:364:20: error: 'fq' may be used uninitialized [-Werror=maybe-uninitialized]
      364 |           y[0] = fq[0]; y[1] = fq[1]; y[2] = fw;
          |                  ~~^~~
    
    This is similar to the warning that is suppressed in the other branch of
    the switch.  Help the compiler knowing that the variable is always
    initialized, which also makes the suppression obsolete.

Diff:
---
 sysdeps/ieee754/dbl-64/k_rem_pio2.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/sysdeps/ieee754/dbl-64/k_rem_pio2.c b/sysdeps/ieee754/dbl-64/k_rem_pio2.c
index 6e2ef5d07b..f75392525c 100644
--- a/sysdeps/ieee754/dbl-64/k_rem_pio2.c
+++ b/sysdeps/ieee754/dbl-64/k_rem_pio2.c
@@ -300,6 +300,11 @@ recompute:
 	iq[jz] = (int32_t) z;
     }
 
+  /* jz is always nonnegative here, because the result is never zero to
+     full precision (this function is not called for zero arguments).
+     Help the compiler to know it.  */
+  if (jz < 0) __builtin_unreachable ();
+
   /* convert integer "bit" chunk to floating-point value */
   fw = __scalbn (one, q0);
   for (i = jz; i >= 0; i--)
@@ -330,16 +335,7 @@ recompute:
       for (i = jz; i >= 0; i--)
 	fv = math_narrow_eval (fv + fq[i]);
       y[0] = (ih == 0) ? fv : -fv;
-      /* GCC mainline (to be GCC 9), as of 2018-05-22 on i686, warns
-	 that fq[0] may be used uninitialized.  This is not possible
-	 because jz is always nonnegative when the above loop
-	 initializing fq is executed, because the result is never zero
-	 to full precision (this function is not called for zero
-	 arguments).  */
-      DIAG_PUSH_NEEDS_COMMENT;
-      DIAG_IGNORE_NEEDS_COMMENT (9, "-Wmaybe-uninitialized");
       fv = math_narrow_eval (fq[0] - fv);
-      DIAG_POP_NEEDS_COMMENT;
       for (i = 1; i <= jz; i++)
 	fv = math_narrow_eval (fv + fq[i]);
       y[1] = (ih == 0) ? fv : -fv;