-Wshift-count-negative in drivers/net/ethernet/sfc/

[nathanchance]

After commit 1e688dd2a3d6 ("powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with asm goto") in the powerpc tree, I see the following warnings with Debian's and SUSE's configs:

$ curl -LSso .config https://github.com/openSUSE/kernel-source/raw/master/config/ppc64le/default

# Remove pahole requirement
$ scripts/config -d BPF_PRELOAD -d DEBUG_INFO_BTF

$ make -skj"$(nproc)" ARCH=powerpc CC=clang CROSS_COMPILE=powerpc64le-linux-gnu- LLVM_IAS=0 olddefconfig drivers/net/ethernet/sfc/
drivers/net/ethernet/sfc/farch.c:985:10: warning: shift count is negative [-Wshift-count-negative]
        WARN_ON(EFX_QWORD_FIELD(*event, FSF_AZ_RX_EV_Q_LABEL) !=
        ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/sfc/bitfield.h:222:26: note: expanded from macro 'EFX_QWORD_FIELD'
#define EFX_QWORD_FIELD         EFX_QWORD_FIELD64
                                ^
drivers/net/ethernet/sfc/bitfield.h:173:2: note: expanded from macro 'EFX_QWORD_FIELD64'
        EFX_EXTRACT_QWORD64(qword, EFX_LOW_BIT(field),          \
        ^
drivers/net/ethernet/sfc/bitfield.h:149:3: note: expanded from macro 'EFX_EXTRACT_QWORD64'
        (EFX_EXTRACT64((qword).u64[0], 0, 63, low, high) &              \
         ^
drivers/net/ethernet/sfc/bitfield.h:134:2: note: expanded from macro 'EFX_EXTRACT64'
        EFX_EXTRACT_NATIVE(le64_to_cpu(element), min, max, low, high)
        ^
drivers/net/ethernet/sfc/bitfield.h:127:20: note: expanded from macro 'EFX_EXTRACT_NATIVE'
         (native_element) << ((min) - (low)))
                          ^
arch/powerpc/include/asm/bug.h:122:30: note: expanded from macro 'WARN_ON'
                                   __label_warn_on, "r" (x));   \
                                   ~~~~~~~~~~~~~~~~~~~~~~^~~
arch/powerpc/include/asm/bug.h:75:7: note: expanded from macro 'WARN_ENTRY'
                  ##__VA_ARGS__ : : label)
                  ~~^~~~~~~~~~~~~~~~~~~~~~
include/linux/compiler_types.h:254:42: note: expanded from macro 'asm_volatile_goto'
#define asm_volatile_goto(x...) asm goto(x)
                                         ^
1 warning generated.
drivers/net/ethernet/sfc/falcon/farch.c:994:10: warning: shift count is negative [-Wshift-count-negative]
        WARN_ON(EF4_QWORD_FIELD(*event, FSF_AZ_RX_EV_Q_LABEL) !=
        ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/sfc/falcon/bitfield.h:222:26: note: expanded from macro 'EF4_QWORD_FIELD'
#define EF4_QWORD_FIELD         EF4_QWORD_FIELD64
                                ^
drivers/net/ethernet/sfc/falcon/bitfield.h:173:2: note: expanded from macro 'EF4_QWORD_FIELD64'
        EF4_EXTRACT_QWORD64(qword, EF4_LOW_BIT(field),          \
        ^
drivers/net/ethernet/sfc/falcon/bitfield.h:149:3: note: expanded from macro 'EF4_EXTRACT_QWORD64'
        (EF4_EXTRACT64((qword).u64[0], 0, 63, low, high) &              \
         ^
drivers/net/ethernet/sfc/falcon/bitfield.h:134:2: note: expanded from macro 'EF4_EXTRACT64'
        EF4_EXTRACT_NATIVE(le64_to_cpu(element), min, max, low, high)
        ^
drivers/net/ethernet/sfc/falcon/bitfield.h:127:20: note: expanded from macro 'EF4_EXTRACT_NATIVE'
         (native_element) << ((min) - (low)))
                          ^
arch/powerpc/include/asm/bug.h:122:30: note: expanded from macro 'WARN_ON'
                                   __label_warn_on, "r" (x));   \
                                   ~~~~~~~~~~~~~~~~~~~~~~^~~
arch/powerpc/include/asm/bug.h:75:7: note: expanded from macro 'WARN_ENTRY'
                  ##__VA_ARGS__ : : label)
                  ~~^~~~~~~~~~~~~~~~~~~~~~
include/linux/compiler_types.h:254:42: note: expanded from macro 'asm_volatile_goto'
#define asm_volatile_goto(x...) asm goto(x)
                                         ^
1 warning generated.

Doing the math, I have no idea how it thinks the shift is going to be negative... (all of the macros are in drivers/net/ethernet/sfc/falcon/bitfield.h)

EF4_QWORD_FIELD(*event, FSF_AZ_RX_EV_Q_LABEL)

EFX_QWORD_FIELD64(*event, FSF_AZ_RX_EV_Q_LABEL)

EF4_EXTRACT_QWORD64(*event, EF4_LOW_BIT(FSF_AZ_RX_EV_Q_LABEL), EF4_HIGH_BIT(FSF_AZ_RX_EV_Q_LABEL))

EF4_EXTRACT_QWORD64(*event, FSF_AZ_RX_EV_Q_LABEL_LBN, FSF_AZ_RX_EV_Q_LABEL_LBN + FSF_AZ_RX_EV_Q_LABEL_WIDTH - 1)

EF4_EXTRACT_QWORD64(*event, 32, 32 + 5 - 1)

EF4_EXTRACT_QWORD64(*event, 32, 36)

EF4_EXTRACT_QWORD64(*event, 32, 36)

EF4_EXTRACT64((*event).u64[0], 0, 63, 32, 36) & EF4_MASK64((36) + 1 - (32))

EF4_EXTRACT64((*event).u64[0], 0, 63, 32, 36) & EF4_MASK64(5)

EF4_EXTRACT64((*event).u64[0], 0, 63, 32, 36) & EF4_MASK64(5)

EF4_EXTRACT_NATIVE(le64_to_cpu((*event).u64[0]), 0, 63, 32, 36) & EF4_MASK64(5)

which finally evaluates to

#define EF4_EXTRACT_NATIVE(native_element, min, max, low, high)		\
	((low) > (max) || (high) < (min) ? 0 :				\
	 (low) > (min) ?						\
	 (native_element) >> ((low) - (min)) :				\
	 (native_element) << ((min) - (low)))

EF4_EXTRACT_NATIVE(le64_to_cpu((*event).u64[0]), 0, 63, 32, 36)
	((32) > (63) || (36) < (0) ? 0 :				\
	 (32) > (0) ?						\
	 (le64_to_cpu((*event).u64[0])) >> ((32) - (0)) :				\
	 (le64_to_cpu((*event).u64[0])) << ((0) - (32)))

Maybe it is trying to evaluate the last branch as a shift of -32? I also do not see what the commit that exposed the problem has to do with this.

[nathanchance]

Looks like that is exactly what is happening. cvise spits out:

$ cat farch.i
void ef4_farch_handle_rx_event() {
  asm goto("" : : "r"(2 ?: 0 << -2) : : __label_warn_on);
__label_warn_on:;
}

$ gcc -O2 -Wall -Wextra -c -o /dev/null farch.i

$ clang -fsyntax-only -Wall -Wextra farch.i
farch.i:2:30: warning: shift count is negative [-Wshift-count-negative]
  asm goto("" : : "r"(2 ?: 0 << -2) : : __label_warn_on);
                             ^  ~~
1 warning generated.

It seems like asm goto is messing this up:

$ cat test.c
cat test.c
int foo(void)
{
        return 1 ? 2 : 0 << -1;
}

void bar(void)
{
        asm("" :: "r"(1 ? 2 : 0 << -1));
}

void baz(void)
{
        asm goto("" :: "r"(1 ? 2 : 0 << -1) :: error);
error:;
}

$ gcc -Wall -Wextra -c -o /dev/null test.c

$ clang -Wall -Wextra -c -o /dev/null test.c
test.c:13:31: warning: shift count is negative [-Wshift-count-negative]
        asm goto("" :: "r"(1 ? 2 : 0 << -1) :: error);
                                     ^  ~~
1 warning generated.

Perhaps this is a similar issue to https://llvm.org/bz51682?

[nickdesaulniers]

llvm/llvm-project#51024

[nickdesaulniers]

https://reviews.llvm.org/D116059