i386: Use a fallback XSAVE layout for remote targets

If a target provides a target description including registers from the
XSAVE extended region, but does not provide an XSAVE layout, use a
fallback XSAVE layout based on the included registers.  This fallback
layout matches GDB's behavior in earlier releases which assumes the
layout from Intel CPUs.

This fallback layout is currently only used for remote targets since
native targets which support XSAVE provide an explicit layout derived
from CPUID.

PR gdb/30912
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30912
Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/i386-tdep.c

@@ -8297,6 +8297,72 @@ i386_floatformat_for_type (struct gdbarch *gdbarch,
   return default_floatformat_for_type (gdbarch, name, len);
 }
 
+/* Compute an XCR0 mask based on a target description.  */
+
+static uint64_t
+i386_xcr0_from_tdesc (const struct target_desc *tdesc)
+{
+  if (! tdesc_has_registers (tdesc))
+    return 0;
+
+  const struct tdesc_feature *feature_core;
+
+  const struct tdesc_feature *feature_sse, *feature_avx, *feature_mpx,
+			     *feature_avx512, *feature_pkeys;
+
+  /* Get core registers.  */
+  feature_core = tdesc_find_feature (tdesc, "org.gnu.gdb.i386.core");
+  if (feature_core == NULL)
+    return 0;
+
+  /* Get SSE registers.  */
+  feature_sse = tdesc_find_feature (tdesc, "org.gnu.gdb.i386.sse");
+
+  /* Try AVX registers.  */
+  feature_avx = tdesc_find_feature (tdesc, "org.gnu.gdb.i386.avx");
+
+  /* Try MPX registers.  */
+  feature_mpx = tdesc_find_feature (tdesc, "org.gnu.gdb.i386.mpx");
+
+  /* Try AVX512 registers.  */
+  feature_avx512 = tdesc_find_feature (tdesc, "org.gnu.gdb.i386.avx512");
+
+  /* Try PKEYS  */
+  feature_pkeys = tdesc_find_feature (tdesc, "org.gnu.gdb.i386.pkeys");
+
+  /* The XCR0 bits.  */
+  uint64_t xcr0 = X86_XSTATE_X87;
+
+  if (feature_sse)
+    xcr0 |= X86_XSTATE_SSE;
+
+  if (feature_avx)
+    {
+      /* AVX register description requires SSE register description.  */
+      if (!feature_sse)
+	return 0;
+
+      xcr0 |= X86_XSTATE_AVX;
+    }
+
+  if (feature_mpx)
+    xcr0 |= X86_XSTATE_MPX_MASK;
+
+  if (feature_avx512)
+    {
+      /* AVX512 register description requires AVX register description.  */
+      if (!feature_avx)
+	return 0;
+
+      xcr0 |= X86_XSTATE_AVX512;
+    }
+
+  if (feature_pkeys)
+    xcr0 |= X86_XSTATE_PKRU;
+
+  return xcr0;
+}
+
 static int
 i386_validate_tdesc_p (i386_gdbarch_tdep *tdep,
 		       struct tdesc_arch_data *tdesc_data)
@@ -8508,6 +8574,15 @@ i386_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
 
   x86_xsave_layout xsave_layout = target_fetch_x86_xsave_layout ();
 
+  /* If the target did not provide an XSAVE layout but the target
+     description includes registers from the XSAVE extended region,
+     use a fallback XSAVE layout.  Specifically, this fallback layout
+     is used when writing out a local core dump for a remote
+     target.  */
+  if (xsave_layout.sizeof_xsave == 0)
+    xsave_layout
+      = i387_fallback_xsave_layout (i386_xcr0_from_tdesc (info.target_desc));
+
   /* If there is already a candidate, use it.  */
   for (arches = gdbarch_list_lookup_by_info (arches, &info);
        arches != NULL;

gdb/i387-tdep.c

@@ -987,6 +987,54 @@ i387_guess_xsave_layout (uint64_t xcr0, size_t xsave_size,
   return true;
 }
 
+/* See i387-tdep.h.  */
+
+x86_xsave_layout
+i387_fallback_xsave_layout (uint64_t xcr0)
+{
+  x86_xsave_layout layout;
+
+  if (HAS_PKRU (xcr0))
+    {
+      /* Intel CPUs supporting PKRU.  */
+      layout.avx_offset = 576;
+      layout.bndregs_offset = 960;
+      layout.bndcfg_offset = 1024;
+      layout.k_offset = 1088;
+      layout.zmm_h_offset = 1152;
+      layout.zmm_offset = 1664;
+      layout.pkru_offset = 2688;
+      layout.sizeof_xsave = 2696;
+    }
+  else if (HAS_AVX512 (xcr0))
+    {
+      /* Intel CPUs supporting AVX512.  */
+      layout.avx_offset = 576;
+      layout.bndregs_offset = 960;
+      layout.bndcfg_offset = 1024;
+      layout.k_offset = 1088;
+      layout.zmm_h_offset = 1152;
+      layout.zmm_offset = 1664;
+      layout.sizeof_xsave = 2688;
+    }
+  else if (HAS_MPX (xcr0))
+    {
+      /* Intel CPUs supporting MPX.  */
+      layout.avx_offset = 576;
+      layout.bndregs_offset = 960;
+      layout.bndcfg_offset = 1024;
+      layout.sizeof_xsave = 1088;
+    }
+  else if (HAS_AVX (xcr0))
+    {
+      /* Intel and AMD CPUs supporting AVX.  */
+      layout.avx_offset = 576;
+      layout.sizeof_xsave = 832;
+    }
+
+  return layout;
+}
+
 /* Extract from XSAVE a bitset of the features that are available on the
    target, but which have not yet been enabled.  */
 

gdb/i387-tdep.h

@@ -147,6 +147,11 @@ extern void i387_supply_fxsave (struct regcache *regcache, int regnum,
 extern bool i387_guess_xsave_layout (uint64_t xcr0, size_t xsave_size,
 				     x86_xsave_layout &layout);
 
+/* Compute an XSAVE layout based on the XCR0 bitmask.  This is used
+   as a fallback if a target does not provide an XSAVE layout.  */
+
+extern x86_xsave_layout i387_fallback_xsave_layout (uint64_t xcr0);
+
 /* Similar to i387_supply_fxsave, but use XSAVE extended state.  */
 
 extern void i387_supply_xsave (struct regcache *regcache, int regnum,