MdePkg/BaseMemoryLibOptDxe ARM: fix arithmetic bugs in CompareMem()

Fix two bugs:
- Erroneous shift of 2 in a bytes to bits conversion.
- Use reverse subtract rather than negate for value that is subsequently
  used as operand #2 in a shift operation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>

diff --git a/MdePkg/Library/BaseMemoryLibOptDxe/Arm/CompareMem.S b/MdePkg/Library/BaseMemoryLibOptDxe/Arm/CompareMem.S
index 951d15777a38d7c41a4f15be754b4fc771ff3b32..3aadebace30f1b18ea1dd982d546c3f826b0d019 100644 (file)
--- a/MdePkg/Library/BaseMemoryLibOptDxe/Arm/CompareMem.S
+++ b/MdePkg/Library/BaseMemoryLibOptDxe/Arm/CompareMem.S
@@ -110,9 +110,9 @@ ASM_PFX(InternalMemCompareMem):
     bic     src1, src1, #3\r
     bic     src2, src2, #3\r
     add     limit, limit, tmp1          // Adjust the limit for the extra.\r
     bic     src1, src1, #3\r
     bic     src2, src2, #3\r
     add     limit, limit, tmp1          // Adjust the limit for the extra.\r

-    lsl     tmp1, tmp1, #2              // Bytes beyond alignment -> bits.\r
+    lsl     tmp1, tmp1, #3              // Bytes beyond alignment -> bits.\r

     ldr     data1, [src1], #4\r
     ldr     data1, [src1], #4\r

-    neg     tmp1, tmp1                  // Bits to alignment -32.\r
+    rsb     tmp1, tmp1, #32             // Bits to alignment -32.\r

     ldr     data2, [src2], #4\r
     mov     tmp2, #~0\r
 \r
     ldr     data2, [src2], #4\r
     mov     tmp2, #~0\r
 \r