Currently, "gcc-4.8.5-28.el7_5.1.x86_64" generates the following code for
me, from the XADD inline assembly added to "X64/GccInline.c" in commit
17634d026f96:
> 0000000000004383 <InternalSyncIncrement>:
> UINT32
> EFIAPI
> InternalSyncIncrement (
> IN volatile UINT32 *Value
> )
> {
> 4383: 55 push %rbp
> 4384: 48 89 e5 mov %rsp,%rbp
> 4387: 48 83 ec 10 sub $0x10,%rsp
> 438b: 48 89 4d 10 mov %rcx,0x10(%rbp)
> UINT32 Result;
>
> __asm__ __volatile__ (
> 438f: 48 8b 55 10 mov 0x10(%rbp),%rdx
> 4393: 48 8b 45 10 mov 0x10(%rbp),%rax
> 4397: b8 01 00 00 00 mov $0x1,%eax
> 439c: f0 0f c1 00 lock xadd %eax,(%rax)
> 43a0: ff c0 inc %eax
> 43a2: 89 45 fc mov %eax,-0x4(%rbp)
> : "m" (*Value) // %2
> : "memory",
> "cc"
> );
>
> return Result;
> 43a5: 8b 45 fc mov -0x4(%rbp),%eax
> }
> 43a8: c9 leaveq
> 43a9: c3 retq
>
The MOV $0X1,%EAX instruction corrupts the address of Value in %RAX before
we reach the XADD instruction. In fact, it makes no sense for XADD to use
%EAX as source operand and (%RAX) as destination operand at the same time.
The XADD instruction's destination operand is a read-write operand. The
GCC documentation states:
> The ordinary output operands must be write-only; GCC will assume that
> the values in these operands before the instruction are dead and need
> not be generated. Extended asm supports input-output or read-write
> operands. Use the constraint character `+' to indicate such an operand
> and list it with the output operands. You should only use read-write
> operands when the constraints for the operand (or the operand in which
> only some of the bits are to be changed) allow a register.
(The above is intentionally quoted from the oldest GCC release that edk2
supports, namely gcc-4.4:
<https://gcc.gnu.org/onlinedocs/gcc-4.4.7/gcc/Extended-Asm.html>.)
Fix the operand list accordingly.
With the patch applied, I get:
> 0000000000004383 <InternalSyncIncrement>:
> UINT32
> EFIAPI
> InternalSyncIncrement (
> IN volatile UINT32 *Value
> )
> {
> 4383: 55 push %rbp
> 4384: 48 89 e5 mov %rsp,%rbp
> 4387: 48 83 ec 10 sub $0x10,%rsp
> 438b: 48 89 4d 10 mov %rcx,0x10(%rbp)
> UINT32 Result;
>
> __asm__ __volatile__ (
> 438f: 48 8b 55 10 mov 0x10(%rbp),%rdx
> 4393: 48 8b 45 10 mov 0x10(%rbp),%rax
> 4397: b8 01 00 00 00 mov $0x1,%eax
> 439c: f0 0f c1 02 lock xadd %eax,(%rdx)
> 43a0: ff c0 inc %eax
> 43a2: 89 45 fc mov %eax,-0x4(%rbp)
> : // no inputs that aren't also outputs
> : "memory",
> "cc"
> );
>
> return Result;
> 43a5: 8b 45 fc mov -0x4(%rbp),%eax
> }
> 43a8: c9 leaveq
> 43a9: c3 retq
Note that some other bugs remain in
"BaseSynchronizationLib/*/GccInline.c"; those should be addressed later,
under <https://bugzilla.tianocore.org/show_bug.cgi?id=1208>.
Cc: Jiewen Yao <[email protected]>
Cc: Liming Gao <[email protected]>
Cc: Michael Kinney <[email protected]>
Cc: Ruiyu Ni <[email protected]>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1207
Fixes: 17634d026f968c404b039a8d8431b6389dd396ea
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <[email protected]>
---
Notes:
Repo: https://github.com/lersek/edk2.git
Branch: xadd_rw
MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c | 12 ++++++------
MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c | 12 ++++++------
2 files changed, 12 insertions(+), 12 deletions(-)
diff --git a/MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c
b/MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c
index d82e0205f553..fa2be7f4b35c 100644
--- a/MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c
+++ b/MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c
@@ -38,11 +38,11 @@ InternalSyncIncrement (
__asm__ __volatile__ (
"movl $1, %%eax \n\t"
"lock \n\t"
- "xadd %%eax, %2 \n\t"
+ "xadd %%eax, %1 \n\t"
"inc %%eax "
: "=a" (Result), // %0
- "=m" (*Value) // %1
- : "m" (*Value) // %2
+ "+m" (*Value) // %1
+ : // no inputs that aren't also outputs
: "memory",
"cc"
);
@@ -75,11 +75,11 @@ InternalSyncDecrement (
__asm__ __volatile__ (
"movl $-1, %%eax \n\t"
"lock \n\t"
- "xadd %%eax, %2 \n\t"
+ "xadd %%eax, %1 \n\t"
"dec %%eax "
: "=a" (Result), // %0
- "=m" (*Value) // %1
- : "m" (*Value) // %2
+ "+m" (*Value) // %1
+ : // no inputs that aren't also outputs
: "memory",
"cc"
);
diff --git a/MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c
b/MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c
index 4c4d6e3fc712..ab7efe23c4db 100644
--- a/MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c
+++ b/MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c
@@ -38,11 +38,11 @@ InternalSyncIncrement (
__asm__ __volatile__ (
"movl $1, %%eax \n\t"
"lock \n\t"
- "xadd %%eax, %2 \n\t"
+ "xadd %%eax, %1 \n\t"
"inc %%eax "
: "=a" (Result), // %0
- "=m" (*Value) // %1
- : "m" (*Value) // %2
+ "+m" (*Value) // %1
+ : // no inputs that aren't also outputs
: "memory",
"cc"
);
@@ -74,11 +74,11 @@ InternalSyncDecrement (
__asm__ __volatile__ (
"movl $-1, %%eax \n\t"
"lock \n\t"
- "xadd %%eax, %2 \n\t"
+ "xadd %%eax, %1 \n\t"
"dec %%eax "
: "=a" (Result), // %0
- "=m" (*Value) // %1
- : "m" (*Value) // %2
+ "+m" (*Value) // %1
+ : // no inputs that aren't also outputs
: "memory",
"cc"
);
--
2.14.1.3.gb7cf6e02401b
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