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| -rw-r--r-- | lib/Target/README.txt | 44 |
1 files changed, 44 insertions, 0 deletions
diff --git a/lib/Target/README.txt b/lib/Target/README.txt index 7fa73ed..f4f767a 100644 --- 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. :) ) + +//===---------------------------------------------------------------------===// |