Add some notes about better flag handling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@41808 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/README.txt

@@ -528,3 +528,48 @@ _foo:
 This apparently occurs in real code.
 
 //===---------------------------------------------------------------------===//
+
+This:
+        #include <algorithm>
+        std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
+        { return std::make_pair(a + b, a + b < a); }
+        bool no_overflow(unsigned a, unsigned b)
+        { return !full_add(a, b).second; }
+
+Should compile to:
+
+_Z8full_addjj:
+	adds	r2, r1, r2
+	movcc	r1, #0
+	movcs	r1, #1
+	str	r2, [r0, #0]
+	strb	r1, [r0, #4]
+	mov	pc, lr
+
+_Z11no_overflowjj:
+	cmn	r0, r1
+	movcs	r0, #0
+	movcc	r0, #1
+	mov	pc, lr
+
+not:
+
+__Z8full_addjj:
+        add r3, r2, r1
+        str r3, [r0]
+        mov r2, #1
+        mov r12, #0
+        cmp r3, r1
+        movlo r12, r2
+        str r12, [r0, #+4]
+        bx lr
+__Z11no_overflowjj:
+        add r3, r1, r0
+        mov r2, #1
+        mov r1, #0
+        cmp r3, r0
+        movhs r1, r2
+        mov r0, r1
+        bx lr
+
+//===---------------------------------------------------------------------===//

lib/Target/PowerPC/README.txt

@@ -677,5 +677,33 @@ LBB1_1: ;bb
         cmplwi cr0, r6, 33920
         bne cr0, LBB1_1
 
-===-------------------------------------------------------------------------===
+//===---------------------------------------------------------------------===//
+
+This:
+        #include <algorithm>
+        inline std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
+        { return std::make_pair(a + b, a + b < a); }
+        bool no_overflow(unsigned a, unsigned b)
+        { return !full_add(a, b).second; }
+
+Should compile to:
+
+__Z11no_overflowjj:
+        add r4,r3,r4
+        subfc r3,r3,r4
+        li r3,0
+        adde r3,r3,r3
+        blr
+
+(or better) not:
+
+__Z11no_overflowjj:
+        add r2, r4, r3
+        cmplw cr7, r2, r3
+        mfcr r2
+        rlwinm r2, r2, 29, 31, 31
+        xori r3, r2, 1
+        blr 
+
+//===---------------------------------------------------------------------===//
 

lib/Target/X86/README.txt

@@ -1178,3 +1178,30 @@ We should sink the load into xmm3 into the LBB1_2 block.  This should
 be pretty easy, and will nuke all the copies.
 
 //===---------------------------------------------------------------------===//
+
+This:
+        #include <algorithm>
+        inline std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
+        { return std::make_pair(a + b, a + b < a); }
+        bool no_overflow(unsigned a, unsigned b)
+        { return !full_add(a, b).second; }
+
+Should compile to:
+
+
+        _Z11no_overflowjj:
+                addl    %edi, %esi
+                setae   %al
+                ret
+
+on x86-64, not:
+
+__Z11no_overflowjj:
+        addl    %edi, %esi
+        cmpl    %edi, %esi
+        setae   %al
+        movzbl  %al, %eax
+        ret
+
+
+//===---------------------------------------------------------------------===//