[glibc] alpha: expect test-float32x-float64-div to fail

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=a32db99a38c5f0c64b414af270830c3b6e3719c1

commit a32db99a38c5f0c64b414af270830c3b6e3719c1
Author: Matt Turner <[email protected]>
Date:   Mon Aug 3 19:55:09 2026 -0400

    alpha: expect test-float32x-float64-div to fail
    
    _Float32x and _Float64 are both binary64 on Alpha, so this narrowing
    divide is a plain divide and the hardware alone decides whether to signal
    underflow.
    
    IEEE 754 determines tininess after rounding from the result rounded as if
    the exponent range were unbounded, while Alpha determines it from the
    delivered result.  The two differ for a quotient that is tiny but rounds
    up to the smallest normal, as in DBL_MIN / (1 + 2^-52) under a rounding
    mode that rounds away from zero: the binade below DBL_MIN has a finer
    spacing than the subnormals, so the unbounded rounding stays below
    DBL_MIN and the result is tiny, but the delivered result is DBL_MIN and
    looks normal.  Alpha signals no underflow for it.
    
    Nothing in software can correct this.  The hardware detects no underflow,
    so no software completion trap is taken and the kernel emulation never
    runs, and as the operation is not really narrowing there is no wider
    intermediate for libm to examine.
    
    Reviewed-by: Adhemerval Zanella  <[email protected]>

Diff:
---
 sysdeps/alpha/Makefile | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/sysdeps/alpha/Makefile b/sysdeps/alpha/Makefile
index 60a369e255..faa59ab27a 100644
--- a/sysdeps/alpha/Makefile
+++ b/sysdeps/alpha/Makefile
@@ -53,6 +53,20 @@ CFLAGS-s_lrint.c += -mieee-with-inexact
 CFLAGS-test-misc.c += -mieee-with-inexact
 # Avoid "conflicting types for built-in function" warnings
 CFLAGS-s_isnan.c += -fno-builtin-isnanf
+
+# _Float32x and _Float64 are both binary64 on Alpha, so this narrowing
+# divide is a plain divide and the hardware alone decides whether to
+# signal underflow.  IEEE 754 determines tininess after rounding from the
+# result rounded as if the exponent range were unbounded, but Alpha
+# determines it from the delivered result.  For a quotient that is tiny
+# but rounds up to the smallest normal -- DBL_MIN / (1 + 2^-52) under a
+# rounding mode that rounds away from zero -- the unbounded rounding
+# stays below DBL_MIN, because that binade has a finer spacing than the
+# subnormals, while the delivered result is DBL_MIN and looks normal.
+# Alpha raises no underflow for it.  No trap is taken, so the kernel
+# emulation cannot correct this, and the operation has no wider
+# intermediate for libm to examine.
+test-xfail-test-float32x-float64-div = yes
 endif
 
 # Build everything with full IEEE math support, and with dynamic rounding;