Add debug support for X86/ELF targets (Linux). This allows llvm-gcc4

[oota-llvm.git] / lib / Target / X86 / README.txt

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index 3ee32cce05581f52c25fd801fc2f328d1e6f733a..956caff0c4e05a4b679e2dacfbb5e6e877972dec 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -607,35 +607,6 @@ or eax, 2
 cmp eax, 6
 jz label
 
-If we aren't going to do this, we should lower the switch better.  We compile 
-the code to:
-
-_f:
-        movl 8(%esp), %eax
-        movl 4(%esp), %ecx
-        cmpl $6, %ecx
-        jl LBB1_4       #entry
-        jmp LBB1_3      #entry
-LBB1_3: #entry
-        cmpl $6, %ecx
-        je LBB1_1       #bb
-        jmp LBB1_2      #UnifiedReturnBlock
-LBB1_4: #entry
-        cmpl $4, %ecx
-        jne LBB1_2      #UnifiedReturnBlock
-LBB1_1: #bb
-        incl %eax
-        ret
-LBB1_2: #UnifiedReturnBlock
-        ret
-
-In the code above, the 'if' is turned into a 'switch' at the mid-level.  It
-looks  like the 'lower to branches' mode could be improved a little here.  In
-particular, the fall-through to LBB1_3 doesn't need a branch.  It would also be
-nice to eliminate the redundant "cmp 6", maybe by lowering to a linear sequence
-of compares if there are below a certain number of cases (instead of a binary
-sequence)?
-
 //===---------------------------------------------------------------------===//
 
 GCC's ix86_expand_int_movcc function (in i386.c) has a ton of interesting
@@ -734,3 +705,30 @@ _foo:
         ret
 
 //===---------------------------------------------------------------------===//
+
+Consider the expansion of:
+
+uint %test3(uint %X) {
+        %tmp1 = rem uint %X, 255
+        ret uint %tmp1
+}
+
+Currently it compiles to:
+
+...
+        movl $2155905153, %ecx
+        movl 8(%esp), %esi
+        movl %esi, %eax
+        mull %ecx
+...
+
+This could be "reassociated" into:
+
+        movl $2155905153, %eax
+        movl 8(%esp), %ecx
+        mull %ecx
+
+to avoid the copy.  In fact, the existing two-address stuff would do this
+except that mul isn't a commutative 2-addr instruction.  I guess this has
+to be done at isel time based on the #uses to mul?
+