[RISCV] Add test for vmerge combine that should be prevented

[luke]

Summary

The fadd in these test cases is constrained and may set fflags differently
depending on the active elements (the nofpexcept flag isn't set on the node).
Therefore to preserve semantics we shouldn't change its mask.

[reames]

LGTM w/comment.

llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll

382

Add a bit to the comment to emphasize that suppressing exceptions is also disallowed by "fpexcept.strict". Removing exceptions is fine under both ignore and maytrap, so it's good to emphasize that bit of the semantic.

llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll

 ; CHECK-NEXT:    vsetvli zero, a0, e32, m1, tu, mu
 ; CHECK-NEXT:    vfadd.vv v8, v9, v10, v0.t
 ; CHECK-NEXT:    ret
   %a = fadd <vscale x 2 x float> %x, %y
   %b = call <vscale x 2 x float> @llvm.vp.merge.nxv2f32(<vscale x 2 x i1> %m, <vscale x 2 x float> %a, <vscale x 2 x float> %passthru, i32 %vl)
   ret <vscale x 2 x float> %b
 }
 
+; This shouldn't be folded because we need to preserve exceptions with
+; "fpexcept.strict" exception behaviour, and masking may hide them.
+define <vscale x 2 x float> @vpmerge_constrained_fadd(<vscale x 2 x float> %passthru, <vscale x 2 x float> %x, <vscale x 2 x float> %y, <vscale x 2 x i1> %m, i64 %vl) strictfp {
+; CHECK-LABEL: vpmerge_constrained_fadd:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli a1, zero, e32, m1, ta, ma
+; CHECK-NEXT:    vfadd.vv v9, v9, v10
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, tu, ma
+; CHECK-NEXT:    vmerge.vvm v8, v8, v9, v0
+; CHECK-NEXT:    ret
+  %a = call <vscale x 2 x float> @llvm.experimental.constrained.fadd(<vscale x 2 x float> %x, <vscale x 2 x float> %y, metadata !"round.dynamic", metadata !"fpexcept.strict") strictfp
+  %b = call <vscale x 2 x float> @llvm.riscv.vmerge.nxv2f32.nxv2f32(<vscale x 2 x float> %passthru, <vscale x 2 x float> %passthru, <vscale x 2 x float> %a, <vscale x 2 x i1> %m, i64 %vl)
+  ret <vscale x 2 x float> %b
+}
+declare <vscale x 2 x float> @llvm.experimental.constrained.fadd(<vscale x 2 x float>, <vscale x 2 x float>, metadata, metadata)
+declare <vscale x 2 x float> @llvm.riscv.vmerge.nxv2f32.nxv2f32(<vscale x 2 x float>, <vscale x 2 x float>, <vscale x 2 x float>, <vscale x 2 x i1>, i64)
+
+; FIXME: This shouldn't be folded because we need to preserve exceptions with
+; "fpexcept.strict" exception behaviour, and masking may hide them.
+define <vscale x 2 x float> @vpmerge_constrained_fadd_vlmax(<vscale x 2 x float> %passthru, <vscale x 2 x float> %x, <vscale x 2 x float> %y, <vscale x 2 x i1> %m) strictfp {
+; CHECK-LABEL: vpmerge_constrained_fadd_vlmax:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli a0, zero, e32, m1, tu, mu
+; CHECK-NEXT:    vfadd.vv v8, v9, v10, v0.t
+; CHECK-NEXT:    ret
+  %a = call <vscale x 2 x float> @llvm.experimental.constrained.fadd(<vscale x 2 x float> %x, <vscale x 2 x float> %y, metadata !"round.dynamic", metadata !"fpexcept.strict") strictfp
+  %b = call <vscale x 2 x float> @llvm.riscv.vmerge.nxv2f32.nxv2f32(<vscale x 2 x float> %passthru, <vscale x 2 x float> %passthru, <vscale x 2 x float> %a, <vscale x 2 x i1> %m, i64 -1)
+  ret <vscale x 2 x float> %b
+}
+
 ; Test conversion by fptosi.
 define <vscale x 2 x i16> @vpmerge_fptosi(<vscale x 2 x i16> %passthru, <vscale x 2 x float> %x, <vscale x 2 x i1> %m, i32 zeroext %vl) {
 ; CHECK-LABEL: vpmerge_fptosi:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetvli zero, a0, e16, mf2, tu, mu
 ; CHECK-NEXT:    vfncvt.rtz.x.f.w v8, v9, v0.t
 ; CHECK-NEXT:    ret
   %a = fptosi <vscale x 2 x float> %x to <vscale x 2 x i16>