[Legalizer] Fix fp-to-uint to fp-tosint promotion assertion.

Summary:
When promoting fp-to-uint16 to fp-to-sint32, the result is actually zero
extended. For example, given double 65534.0, without legalization:

  fp-to-uint16: 65534.0 -> 0xfffe

With the legalization:

  fp-to-sint32: 65534.0 -> 0x0000fffe

Without this patch, legalization wrongly emits a signed extend assertion,
which is consumed by later icmp instruction, and cause miscompile.

Note that the floating point value must be in [0, 65535), otherwise the
behavior is undefined.

This patch reverts r279223 behavior and adds more tests and
documentations.

In PR29041's context, James Molloy mentioned that:

  We don't need to mask because conversion from float->uint8_t is
  undefined if the integer part of the float value is not representable in
  uint8_t. Therefore we can assume this doesn't happen!

which is totally true and good, because fptoui is documented clearly to
have undefined behavior when overflow/underflow happens. We should take
the advantage of this behavior so that we can save unnecessary mask
instructions.

Reviewers: jmolloy, nadav, echristo, kbarton

Subscribers: mehdi_amini, nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D28284

llvm-svn: 291015

1 files changed, 21 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/PowerPC/fp64-to-int16.ll b/llvm/test/CodeGen/PowerPC/fp64-to-int16.ll
new file mode 100644
index 00000000000..10d58c2d766
--- /dev/null
+++ b/llvm/test/CodeGen/PowerPC/fp64-to-int16.ll
@@ -0,0 +1,21 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -O0 < %s | FileCheck %s
+target triple = "powerpc64le--linux-gnu"
+
+define i1 @Test(double %a) {
+; CHECK-LABEL: Test:
+; CHECK:       # BB#0: # %entry
+; CHECK-NEXT:    xscvdpsxws 1, 1
+; CHECK-NEXT:    mfvsrwz 3, 1
+; CHECK-NEXT:    xori 3, 3, 65534
+; CHECK-NEXT:    cntlzw 3, 3
+; CHECK-NEXT:    srwi 3, 3, 5
+; CHECK-NEXT:    # implicit-def: %X4
+; CHECK-NEXT:    mr 4, 3
+; CHECK-NEXT:    mr 3, 4
+; CHECK-NEXT:    blr
+entry:
+  %conv = fptoui double %a to i16
+  %cmp = icmp eq i16 %conv, -2
+  ret i1 %cmp
+}