exec: Build page-vary-common.c with -fno-lto

In bbc17caf81f, we used an alias attribute to allow target_page
to be declared const, and yet be initialized late.

This fails when using LTO with several versions of gcc.
The compiler looks through the alias and decides that the const
variable is statically initialized to zero, then propagates that
zero to many uses of the variable.

This can be avoided by compiling one object file with -fno-lto.
In this way, any initializer cannot be seen, and the constant
propagation does not occur.

Since we are certain to have this separate compilation unit, we
can drop the alias attribute as well.  We simply have differing
declarations for target_page in different compilation units.
Drop the use of init_target_page, and drop the configure detection
for CONFIG_ATTRIBUTE_ALIAS.

In order to change the compilation flags for a file with meson,
we must use a static_library.  This runs into specific_ss, where
we would need to create many static_library instances.

Fix this by splitting page-vary.c: the page-vary-common.c part is
compiled once as a static_library, while the page-vary.c part is
left in specific_ss in order to handle the target-specific value
of TARGET_PAGE_BITS_MIN.

Reported-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210321211534.2101231-1-richard.henderson@linaro.org>
[PMD: Fix typo in subject, split original patch in 3]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Gavin Shan <gshan@redhat.com>
Message-Id: <20210322112427.4045204-4-f4bug@amsat.org>
[rth: Update MAINTAINERS]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/meson.build b/meson.build
index f0dd8aa089..c6f4b0cf5e 100644
--- a/meson.build
+++ b/meson.build
@@ -1944,6 +1944,24 @@ specific_ss.add(when: 'CONFIG_TCG', if_true: files(
 ))
 specific_ss.add(when: 'CONFIG_TCG_INTERPRETER', if_true: files('tcg/tci.c'))
 
+# Work around a gcc bug/misfeature wherein constant propagation looks
+# through an alias:
+#   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99696
+# to guess that a const variable is always zero.  Without lto, this is
+# impossible, as the alias is restricted to page-vary-common.c.  Indeed,
+# without lto, not even the alias is required -- we simply use different
+# declarations in different compilation units.
+pagevary = files('page-vary-common.c')
+if get_option('b_lto')
+  pagevary_flags = ['-fno-lto']
+  if get_option('cfi')
+    pagevary_flags += '-fno-sanitize=cfi-icall'
+  endif
+  pagevary = static_library('page-vary-common', sources: pagevary,
+                            c_args: pagevary_flags)
+  pagevary = declare_dependency(link_with: pagevary)
+endif
+common_ss.add(pagevary)
 specific_ss.add(files('page-vary.c'))
 
 subdir('backends')