From e6459b9e6c9fc84414c87631df3662ce9cfbeabc Mon Sep 17 00:00:00 2001 From: Ruiyu Ni Date: Wed, 7 Nov 2018 23:51:51 +0800 Subject: MdePkg/BaseSynchronizationLib: Fix InternalSync[De|In]crement MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1303 Today's code generates assembly code as below for InternalSyncIncrement: __asm__ __volatile__ ( "movl $1, %%eax \n\t" "lock \n\t" "xadd %%eax, %1 \n\t" "inc %%eax \n\t" : "=a" (Result), // %0 "+m" (*Value) // %1 : // no inputs that aren't also outputs : "memory", "cc" ); 0: 55 pushl %ebp 1: 89 e5 movl %esp, %ebp 3: 8b 45 08 movl 8(%ebp), %eax 6: b8 01 00 00 00 movl $1, %eax b: f0 lock c: 0f c1 00 xaddl %eax, _InternalSyncIncrement(%eax) f: 40 incl %eax 10: 5d popl %ebp 11: c3 retl Line #3 and Line #6 both use EAX as destination register. Line #c uses EAX and (EAX). The output operand "=a" tells GCC that EAX is used for output. But GCC only assumes that EAX will be used in the very last instruction. Per GCC document, "Use the '&' constraint modifier on all output operands that must not overlap an input. Otherwise, GCC may allocate the output operand in the same register as an unrelated input operand, on the assumption that the assembler code consumes its inputs before producing outputs. This assumption may be false if the assembler code actually consists of more than one instruction." "=&a" should be used to tell GCC not use EAX before the assembly. Contributed-under: TianoCore Contribution Agreement 1.1 Signed-off-by: Ruiyu Ni Cc: Liming Gao Cc: Michael Kinney Cc: Laszlo Ersek Cc: Philippe Mathieu-Daudé Cc: Ard Biesheuvel Reviewed-by: Laszlo Ersek Tested-by: Laszlo Ersek Suggested-by: Ard Biesheuvel Fixes: 8a94eb9283fa09a30f5f06f0c12cf0ee4e14fbcf Fixes: 17634d026f968c404b039a8d8431b6389dd396ea --- MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c | 4 ++-- MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c | 4 ++-- 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c b/MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c index af39bdeb51..760a020a32 100644 --- a/MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c +++ b/MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c @@ -40,7 +40,7 @@ InternalSyncIncrement ( "lock \n\t" "xadd %%eax, %1 \n\t" "inc %%eax \n\t" - : "=a" (Result), // %0 + : "=&a" (Result), // %0 "+m" (*Value) // %1 : // no inputs that aren't also outputs : "memory", @@ -76,7 +76,7 @@ InternalSyncDecrement ( "lock \n\t" "xadd %%eax, %1 \n\t" "dec %%eax \n\t" - : "=a" (Result), // %0 + : "=&a" (Result), // %0 "+m" (*Value) // %1 : // no inputs that aren't also outputs : "memory", diff --git a/MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c b/MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c index edb904c007..767d4626b8 100644 --- a/MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c +++ b/MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c @@ -40,7 +40,7 @@ InternalSyncIncrement ( "lock \n\t" "xadd %%eax, %1 \n\t" "inc %%eax \n\t" - : "=a" (Result), // %0 + : "=&a" (Result), // %0 "+m" (*Value) // %1 : // no inputs that aren't also outputs : "memory", @@ -76,7 +76,7 @@ InternalSyncDecrement ( "lock \n\t" "xadd %%eax, %1 \n\t" "dec %%eax \n\t" - : "=a" (Result), // %0 + : "=&a" (Result), // %0 "+m" (*Value) // %1 : // no inputs that aren't also outputs : "memory", -- cgit