[APFloat] Set size of PPCDoubleDouble to 128

[qiucf]

Summary

Commit 566690b0 uses size information in float semantics, but PPCDoubleDouble left them empty.

As follow-up, we can consider remove PPCDoubleDoubleLegacy and fill other fields in the future.

[foad]

LGTM. I didn't see any breakage from my patch so I guess ppcf128_ones_store is a new test?

[qiucf]

I didn't see any breakage from my patch so I guess ppcf128_ones_store is a new test?

Yes, I added that by a staged diff :-)

llvm/lib/Support/APFloat.cpp

/* The IBM double-double semantics. Such a number consists of a pair of IEEE
      64-bit doubles (Hi, Lo), where |Hi| > |Lo|, and if normal,
      (double)(Hi + Lo) == Hi. The numeric value it's modeling is Hi + Lo.
      Therefore it has two 53-bit mantissa parts that aren't necessarily adjacent
      to each other, and two 11-bit exponents.
 
      Note: we need to make the value different from semBogus as otherwise
      an unsafe optimization may collapse both values to a single address,
      and we heavily rely on them having distinct addresses.             */
-  static const fltSemantics semPPCDoubleDouble = {-1, 0, 0, 0};
+  static const fltSemantics semPPCDoubleDouble = {-1, 0, 0, 128};
 
   /* These are legacy semantics for the fallback, inaccrurate implementation of
      IBM double-double, if the accurate semPPCDoubleDouble doesn't handle the
      operation. It's equivalent to having an IEEE number with consecutive 106
      bits of mantissa and 11 bits of exponent.
 
      It's not equivalent to IBM double-double. For example, a legit IBM
      double-double, 1 + epsilon:

llvm/test/Transforms/InstCombine/bitcast-store.ll

 %swift.error = type opaque
 define void @swifterror_store(i64* %x, %swift.error** swifterror %err) {
 entry:
   %casted = bitcast i64* %x to %swift.error*
   store %swift.error* %casted, %swift.error** %err
   ret void
 }
 
+; CHECK-LABEL: @ppcf128_ones_store
+; CHECK: store ppc_fp128 0xMFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF, ppc_fp128* %dest, align 16
+define void @ppcf128_ones_store(ppc_fp128* %dest) {
+entry:
+  %int = or i128 0, 340282366920938463463374607431768211455 ; 128 ones
+  %val = bitcast i128 %int to ppc_fp128
+  store ppc_fp128 %val, ppc_fp128* %dest, align 16
+  ret void
+}
+
 !0 = !{!1}
 !1 = !{!1, !2}
 !2 = !{!2}
 !3 = !{}