[newlib-cygwin/main] Fix GCN SIMD libm bug

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=2ef1a37e7823b21eda524972c006e0e8c26f97b3

commit 2ef1a37e7823b21eda524972c006e0e8c26f97b3
Author:     Andrew Stubbs <ams@baylibre.com>
AuthorDate: Thu Mar 20 09:32:31 2025 +0000
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Tue Mar 25 12:24:16 2025 +0100

    Fix GCN SIMD libm bug
    
    Since January, GCC has been miscompiling Newlib libm on AMD GCN due to
    undefined behaviour in the RESIZE_VECTOR macro.  It was "working" but expanding
    the size of a vector would no longer zero the additional lanes, as it expected.
    
    See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119325

Diff:
---
 newlib/libm/machine/amdgcn/amdgcn_veclib.h | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/newlib/libm/machine/amdgcn/amdgcn_veclib.h b/newlib/libm/machine/amdgcn/amdgcn_veclib.h
index bd67740ac3ab..9e9d3ebf025d 100644
--- a/newlib/libm/machine/amdgcn/amdgcn_veclib.h
+++ b/newlib/libm/machine/amdgcn/amdgcn_veclib.h
@@ -85,8 +85,21 @@ typedef union {
 
 #define RESIZE_VECTOR(to_t, from) \
 ({ \
-  __auto_type __from = (from); \
-  *((to_t *) &__from); \
+  to_t __to; \
+  if (VECTOR_WIDTH (to_t) < VECTOR_WIDTH (__typeof (from))) \
+    asm ("; no-op cast %0" : "=v"(__to) : "0"(from)); \
+  else \
+    { \
+      unsigned long __mask = -1L; \
+      int lanes = VECTOR_WIDTH (__typeof (from)); \
+      __mask <<= lanes; \
+      __builtin_choose_expr ( \
+	V_SF_SI_P (to_t), \
+	({asm ("v_mov_b32 %0, 0" : "=v"(__to) : "0"(from), "e"(__mask));}), \
+	({asm ("v_mov_b32 %H0, 0\n\t" \
+	       "v_mov_b32 %L0, 0" : "=v"(__to) : "0"(from), "e"(__mask));})); \
+    } \
+  __to; \
 })
 
 /* Bit-wise cast vector FROM to type TO_T.  */