Add README entry; based on testcase from Bill Hart.

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

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 7fa73edaefee2509128c39beb0e591949c7f4c87..f4f767aa4d5f00ae8a762c18d3bc06038297aebe 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1863,3 +1863,47 @@ LLVM prefers comparisons with zero over non-zero in general, but in this
 case it choses instead to keep the max operation obvious.
 
 //===---------------------------------------------------------------------===//
+
+Take the following testcase on x86-64 (similar testcases exist for all targets
+with addc/adde):
+
+define void @a(i64* nocapture %s, i64* nocapture %t, i64 %a, i64 %b,
+i64 %c) nounwind {
+entry:
+ %0 = zext i64 %a to i128                        ; <i128> [#uses=1]
+ %1 = zext i64 %b to i128                        ; <i128> [#uses=1]
+ %2 = add i128 %1, %0                            ; <i128> [#uses=2]
+ %3 = zext i64 %c to i128                        ; <i128> [#uses=1]
+ %4 = shl i128 %3, 64                            ; <i128> [#uses=1]
+ %5 = add i128 %4, %2                            ; <i128> [#uses=1]
+ %6 = lshr i128 %5, 64                           ; <i128> [#uses=1]
+ %7 = trunc i128 %6 to i64                       ; <i64> [#uses=1]
+ store i64 %7, i64* %s, align 8
+ %8 = trunc i128 %2 to i64                       ; <i64> [#uses=1]
+ store i64 %8, i64* %t, align 8
+ ret void
+}
+
+Generated code:
+       addq    %rcx, %rdx
+       movl    $0, %eax
+       adcq    $0, %rax
+       addq    %r8, %rax
+       movq    %rax, (%rdi)
+       movq    %rdx, (%rsi)
+       ret
+
+Expected code:
+       addq    %rcx, %rdx
+       adcq    $0, %r8
+       movq    %r8, (%rdi)
+       movq    %rdx, (%rsi)
+       ret
+
+The generated SelectionDAG has an ADD of an ADDE, where both operands of the
+ADDE are zero. Replacing one of the operands of the ADDE with the other operand
+of the ADD, and replacing the ADD with the ADDE, should give the desired result.
+
+(That said, we are doing a lot better than gcc on this testcase. :) )
+
+//===---------------------------------------------------------------------===//