idiom recognition should catch this.
authorChris Lattner <sabre@nondot.org>
Sun, 19 Sep 2010 00:37:34 +0000 (00:37 +0000)
committerChris Lattner <sabre@nondot.org>
Sun, 19 Sep 2010 00:37:34 +0000 (00:37 +0000)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114304 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/README.txt

index 4faf8bcfd419982b438c73c5758e88e8f6840cce..32e7385d23a793c944ee40661c318af74ae48f17 100644 (file)
@@ -2,6 +2,38 @@ Target Independent Opportunities:
 
 //===---------------------------------------------------------------------===//
 
+We should recognize idioms for add-with-carry and turn it into the appropriate
+intrinsics.  This example:
+
+unsigned add32carry(unsigned sum, unsigned x) {
+ unsigned z = sum + x;
+ if (sum + x < x)
+     z++;
+ return z;
+}
+
+Compiles to: clang t.c -S -o - -O3 -fomit-frame-pointer -m64 -mkernel
+
+_add32carry:                            ## @add32carry
+       addl    %esi, %edi
+       cmpl    %esi, %edi
+       sbbl    %eax, %eax
+       andl    $1, %eax
+       addl    %edi, %eax
+       ret
+
+with clang, but to:
+
+_add32carry:
+       leal    (%rsi,%rdi), %eax
+       cmpl    %esi, %eax
+       adcl    $0, %eax
+       ret
+
+with gcc.
+
+//===---------------------------------------------------------------------===//
+
 Dead argument elimination should be enhanced to handle cases when an argument is
 dead to an externally visible function.  Though the argument can't be removed
 from the externally visible function, the caller doesn't need to pass it in.