[llvm] r296865 - [ARM] fpscr read/write intrinsics not aware of each other

[Ranjeet Singh via llvm-commits]

Author: rsingh
Date: Fri Mar  3 05:40:07 2017
New Revision: 296865

URL: http://llvm.org/viewvc/llvm-project?rev=296865&view=rev
Log:
[ARM] fpscr read/write intrinsics not aware of each other

The intrinsics __builtin_arm_get_fpscr and __builtin_arm_set_fpscr read and
write to the fpscr (Floating-Point Status and Control Register) register.

A bug exists in the __builtin_arm_get_fpscr intrinsic definition in llvm which
treats this intrinsic as a IntroNoMem which means it's not a memory access and
doesn't have any other side-effects. Having this property on this intrinsic
means that various optimizations can be done on this such as common
sub-expression elimination with other reads. This can cause issues if there has
been write to this register, e.g.

void foo(int *p) {
     p[0] = __builtin_arm_get_fpscr();
     __builtin_arm_set_fpscr(1);
     p[1] = __builtin_arm_get_fpscr();
}

in the above example the second read is currently CSE'd into the first read,
this is because llvm isn't aware that the write done by __builtin_arm_set_fpscr
effects the same register that __builtin_arm_get_fpscr reads from, to fix this
problem I've removed the property IntrNoMem so that __builtin_arm_get_fpscr is
treated as a memory access.

Differential Revision: https://reviews.llvm.org/D30542


Added:
    llvm/trunk/test/CodeGen/ARM/fpscr-intrinsics.ll
Modified:
    llvm/trunk/include/llvm/IR/IntrinsicsARM.td

Modified: llvm/trunk/include/llvm/IR/IntrinsicsARM.td
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/IntrinsicsARM.td?rev=296865&r1=296864&r2=296865&view=diff
==============================================================================
--- llvm/trunk/include/llvm/IR/IntrinsicsARM.td (original)
+++ llvm/trunk/include/llvm/IR/IntrinsicsARM.td Fri Mar  3 05:40:07 2017
@@ -67,7 +67,7 @@ def int_arm_isb : GCCBuiltin<"__builtin_
 // VFP
 
 def int_arm_get_fpscr : GCCBuiltin<"__builtin_arm_get_fpscr">,
-                       Intrinsic<[llvm_i32_ty], [], [IntrNoMem]>;
+                       Intrinsic<[llvm_i32_ty], [], []>;
 def int_arm_set_fpscr : GCCBuiltin<"__builtin_arm_set_fpscr">,
                        Intrinsic<[], [llvm_i32_ty], []>;
 def int_arm_vcvtr     : Intrinsic<[llvm_float_ty], [llvm_anyfloat_ty],

Added: llvm/trunk/test/CodeGen/ARM/fpscr-intrinsics.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/ARM/fpscr-intrinsics.ll?rev=296865&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/ARM/fpscr-intrinsics.ll (added)
+++ llvm/trunk/test/CodeGen/ARM/fpscr-intrinsics.ll Fri Mar  3 05:40:07 2017
@@ -0,0 +1,23 @@
+; RUN: llc < %s -O0 -mtriple=armv7-eabi -mcpu=cortex-a8 | FileCheck %s
+; RUN: llc < %s -O3 -mtriple=armv7-eabi -mcpu=cortex-a8 | FileCheck %s
+
+; Function Attrs: nounwind
+define void @fn1(i32* nocapture %p) local_unnamed_addr {
+entry:
+  ; CHECK: vmrs r{{[0-9]+}}, fpscr
+  %0 = tail call i32 @llvm.arm.get.fpscr()
+  store i32 %0, i32* %p, align 4
+  ; CHECK: vmsr fpscr, r{{[0-9]+}}
+  tail call void @llvm.arm.set.fpscr(i32 1)
+  ; CHECK: vmrs r{{[0-9]+}}, fpscr
+  %1 = tail call i32 @llvm.arm.get.fpscr()
+  %arrayidx1 = getelementptr inbounds i32, i32* %p, i32 1
+  store i32 %1, i32* %arrayidx1, align 4
+  ret void
+}
+
+; Function Attrs: nounwind readonly
+declare i32 @llvm.arm.get.fpscr()
+
+; Function Attrs: nounwind writeonly
+declare void @llvm.arm.set.fpscr(i32)