https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103775
Bug ID: 103775
Summary: [12 Regression] Assembler messages: Warning:
unpredictable transfer with writeback -- `ldrb
w0,[x0,16]!'
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: zsojka at seznam dot cz
Target Milestone: ---
Host: x86_64-pc-linux-gnu
Target: aarch64-unknown-linux-gnu
Created attachment 52035
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52035&action=edit
reduced testcase
According to what I could find:
ARM DDI 0487G.b (ID072021)
K1.2.16
CONSTRAINED UNPREDICTABLE behavior for A64 instructions
the behavior of the generated instruction is not guaranteed to be what the
compiler might expected.
Compiler output:
$ aarch64-unknown-linux-gnu-gcc -Og -fno-forward-propagate -free testcase.c -c
/tmp/ccOyWfMV.s: Assembler messages:
/tmp/ccOyWfMV.s:11: Warning: unpredictable transfer with writeback -- `ldrb
w0,[x0,16]!'
$ aarch64-unknown-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-aarch64/bin/aarch64-unknown-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-r12-6072-20211220084313-g8d1e342b4af-checking-yes-rtl-df-extra-aarch64/bin/../libexec/gcc/aarch64-unknown-linux-gnu/12.0.0/lto-wrapper
Target: aarch64-unknown-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--with-cloog --with-ppl --with-isl
--with-sysroot=/usr/aarch64-unknown-linux-gnu --build=x86_64-pc-linux-gnu
--host=x86_64-pc-linux-gnu --target=aarch64-unknown-linux-gnu
--with-ld=/usr/bin/aarch64-unknown-linux-gnu-ld
--with-as=/usr/bin/aarch64-unknown-linux-gnu-as --disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-r12-6072-20211220084313-g8d1e342b4af-checking-yes-rtl-df-extra-aarch64
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0 20211220 (experimental) (GCC)
