Add intrinsics for constrained floating point operations

This commit introduces a set of experimental intrinsics intended to prevent
optimizations that make assumptions about the rounding mode and floating point
exception behavior.  These intrinsics will later be extended to specify
flush-to-zero behavior.  More work is also required to model instruction
dependencies in machine code and to generate these instructions from clang
(when required by pragmas and/or command line options that are not currently
supported).

Differential Revision: https://reviews.llvm.org/D27028

llvm-svn: 293226

1 files changed, 102 insertions, 0 deletions

diff --git a/llvm/test/Feature/fp-intrinsics.ll b/llvm/test/Feature/fp-intrinsics.ll
new file mode 100644
index 00000000000..960bfb5ca10
--- /dev/null
+++ b/llvm/test/Feature/fp-intrinsics.ll
@@ -0,0 +1,102 @@
+; RUN: opt -O3 -S < %s | FileCheck %s
+
+; Test to verify that constants aren't folded when the rounding mode is unknown.
+; CHECK-LABEL: @f1
+; CHECK: call double @llvm.experimental.constrained.fdiv.f64
+define double @f1() {
+entry:
+  %div = call double @llvm.experimental.constrained.fdiv.f64(
+                                               double 1.000000e+00,
+                                               double 1.000000e+01,
+                                               metadata !"round.dynamic",
+                                               metadata !"fpexcept.strict")
+  ret double %div
+}
+
+; Verify that 'a - 0' isn't simplified to 'a' when the rounding mode is unknown.
+;
+; double f2(double a) {
+;   // Because the result of '0 - 0' is negative zero if rounding mode is
+;   // downward, this shouldn't be simplified.
+;   return a - 0.0;
+; }
+;
+; CHECK-LABEL: @f2
+; CHECK: call double @llvm.experimental.constrained.fsub.f64
+define double @f2(double %a) {
+entry:
+  %div = call double @llvm.experimental.constrained.fsub.f64(
+                                               double %a, double 0.000000e+00,
+                                               metadata !"round.dynamic",
+                                               metadata !"fpexcept.strict")
+  ret double %div
+}
+
+; Verify that '-((-a)*b)' isn't simplified to 'a*b' when the rounding mode is
+; unknown.
+;
+; double f3(double a, double b) {
+;   // Because the intermediate value involved in this calculation may require
+;   // rounding, this shouldn't be simplified.
+;   return -((-a)*b);
+; }
+;
+; CHECK-LABEL: @f3
+; CHECK: call double @llvm.experimental.constrained.fsub.f64
+; CHECK: call double @llvm.experimental.constrained.fmul.f64
+; CHECK: call double @llvm.experimental.constrained.fsub.f64
+define double @f3(double %a, double %b) {
+entry:
+  %sub = call double @llvm.experimental.constrained.fsub.f64(
+                                               double -0.000000e+00, double %a,
+                                               metadata !"round.dynamic",
+                                               metadata !"fpexcept.strict")
+  %mul = call double @llvm.experimental.constrained.fmul.f64(
+                                               double %sub, double %b,
+                                               metadata !"round.dynamic",
+                                               metadata !"fpexcept.strict")
+  %ret = call double @llvm.experimental.constrained.fsub.f64(
+                                               double -0.000000e+00,
+                                               double %mul,
+                                               metadata !"round.dynamic",
+                                               metadata !"fpexcept.strict")
+  ret double %ret
+}
+
+; Verify that FP operations are not performed speculatively when FP exceptions
+; are not being ignored.
+;
+; double f4(int n, double a) {
+;   // Because a + 1 may overflow, this should not be simplified.
+;   if (n > 0)
+;     return a + 1.0;
+;   return a;
+; }
+;
+; 
+; CHECK-LABEL: @f4
+; CHECK-NOT: select
+; CHECK: br i1 %cmp
+define double @f4(i32 %n, double %a) {
+entry:
+  %cmp = icmp sgt i32 %n, 0
+  br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+  %add = call double @llvm.experimental.constrained.fadd.f64(
+                                               double 1.000000e+00, double %a,
+                                               metadata !"round.dynamic",
+                                               metadata !"fpexcept.strict")
+  br label %if.end
+
+if.end:
+  %a.0 = phi double [%add, %if.then], [ %a, %entry ]
+  ret double %a.0
+}
+
+
+@llvm.fp.env = thread_local global i8 zeroinitializer, section "llvm.metadata"
+declare double @llvm.experimental.constrained.fdiv.f64(double, double, metadata, metadata)
+declare double @llvm.experimental.constrained.fmul.f64(double, double, metadata, metadata)
+declare double @llvm.experimental.constrained.fadd.f64(double, double, metadata, metadata)
+declare double @llvm.experimental.constrained.fsub.f64(double, double, metadata, metadata)