mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2025-02-01 01:43:57 +00:00
a125fb1689
comparison with 0. These two pieces of code should give identical results: rsbs r1, r1, 0 cmp r0, r1 mov r0, #0 it ls mov r0, #1 and: cmn r0, r1 mov r0, #0 it ls mov r0, #1 However, the CMN gives the *opposite* result when r1 is 0. This is because the carry flag is set in the CMP case but not in the CMN case. In short, the CMP instruction doesn't perform a truncate of the (logical) NOT of 0 plus the value of r0 and the carry bit (because the "carry bit" parameter to AddWithCarry is defined as 1 in this case, the carry flag will always be set when r0 >= 0). The CMN instruction doesn't perform a NOT of 0 so there is never a "carry" when this AddWithCarry is performed (because the "carry bit" parameter to AddWithCarry is defined as 0). The AddWithCarry in the CMP case seems to be relying upon the identity: ~x + 1 = -x However when x is 0 and unsigned, this doesn't hold: x = 0 ~x = 0xFFFF FFFF ~x + 1 = 0x1 0000 0000 (-x = 0) != (0x1 0000 0000 = ~x + 1) Therefore, we should disable *all* versions of CMN, especially when comparing against zero, until we can limit when the CMN instruction is used (when we know that the RHS is not 0) or when we have a hardware fix for this. (See the ARM docs for the "AddWithCarry" pseudo-code.) This is related to <rdar://problem/7569620>. llvm-svn: 112176
\Low Level Virtual Machine (LLVM) ================================ This directory and its subdirectories contain source code for the Low Level Virtual Machine, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the HTML documentation provided in docs/index.html for further assistance with LLVM. If you're writing a package for LLVM, see docs/Packaging.html for our suggestions.