[X86][ABI] Do not return float/double from x87 registers when x87 is disabled

[pengfei]

Summary

This is aligned with GCC's behavior.
Also, alias -mno-fp-ret-in-387 to -mno-x87, by which we can fix pr51498.

[pengfei]

Found a mistake, working on that.

[pengfei]

Fix bugs. Ready for review now.

[nickdesaulniers]

Fixes https://bugs.llvm.org/show_bug.cgi?id=51498?

[pengfei]

In D112143#3089072, @nickdesaulniers wrote:

Fixes https://bugs.llvm.org/show_bug.cgi?id=51498?

Yes. I mentioned pr51498 in commit message :)

[nickdesaulniers]

In D112143#3089072, @nickdesaulniers wrote:

Fixes https://bugs.llvm.org/show_bug.cgi?id=51498?

sorry, I missed the PR # in the description; my mistake.

[nickdesaulniers]

clang/test/Driver/x86-target-features.c

5

If you split this test and the above change to clang/include/clang/Driver/Options.td into a separate patch on Phab, I'd be happy to accept that!

Otherwise someone more familiar with x86 will have to take a look at the below changes.

[pengfei]

clang/test/Driver/x86-target-features.c

5

This is a simple alias. Spliting it doesn't solve the initial problem. I think it's better to commit it together with the backend change.

[nickdesaulniers]

Just a comment on the ECX bit, otherwise this patch looks fine to me. Someone with more x86 knowledge may want to take a look though. Thanks for the patch!

llvm/lib/Target/X86/X86CallingConv.td

283

Is there a test case that exercises the assignment to %ecx?

[pengfei]

1. Add support for f80.
2. Add test case for 3 return values.

[pengfei]

llvm/lib/Target/X86/X86CallingConv.td

283

Add one test case for it, though I don't believe we can really generate it from frontend.

[nickdesaulniers]

llvm/test/CodeGen/X86/no-ret-in-x87-reg.ll

147–150 ↗

(On Diff #387855)

:(

[pengfei]

Thanks for the review!

llvm/test/CodeGen/X86/no-ret-in-x87-reg.ll

147–150 ↗

(On Diff #387855)

Will fix it as a follow up. D100091 aims to fix these problems.

clang/include/clang/Driver/Options.td

foreach i = {0-7} in
   def ffixed_d#i : Flag<["-"], "ffixed-d"#i>, Group<m_m68k_Features_Group>,
     HelpText<"Reserve the d"#i#" register (M68k only)">;
 
 // X86 feature flags
 def mx87 : Flag<["-"], "mx87">, Group<m_x86_Features_Group>;
 def mno_x87 : Flag<["-"], "mno-x87">, Group<m_x86_Features_Group>;
 def m80387 : Flag<["-"], "m80387">, Alias<mx87>;
 def mno_80387 : Flag<["-"], "mno-80387">, Alias<mno_x87>;
+def mno_fp_ret_in_387 : Flag<["-"], "mno-fp-ret-in-387">, Alias<mno_x87>;
 def mmmx : Flag<["-"], "mmmx">, Group<m_x86_Features_Group>;
 def mno_mmx : Flag<["-"], "mno-mmx">, Group<m_x86_Features_Group>;
 def m3dnow : Flag<["-"], "m3dnow">, Group<m_x86_Features_Group>;
 def mno_3dnow : Flag<["-"], "mno-3dnow">, Group<m_x86_Features_Group>;
 def m3dnowa : Flag<["-"], "m3dnowa">, Group<m_x86_Features_Group>;
 def mno_3dnowa : Flag<["-"], "mno-3dnowa">, Group<m_x86_Features_Group>;
 def mamx_bf16 : Flag<["-"], "mamx-bf16">, Group<m_x86_Features_Group>;
 def mno_amx_bf16 : Flag<["-"], "mno-amx-bf16">, Group<m_x86_Features_Group>;

clang/test/Driver/x86-target-features.c

 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mx87 %s -### -o %t.o 2>&1 | FileCheck -check-prefix=X87 %s
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mno-x87 %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-X87 %s
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -m80387 %s -### -o %t.o 2>&1 | FileCheck -check-prefix=X87 %s
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mno-80387 %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-X87 %s
+// RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mno-fp-ret-in-387 %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-X87 %s
 // X87: "-target-feature" "+x87"
 // NO-X87: "-target-feature" "-x87"
 
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mmmx -m3dnow -m3dnowa %s -### -o %t.o 2>&1 | FileCheck -check-prefix=MMX %s
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mno-mmx -mno-3dnow -mno-3dnowa %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-MMX %s
 // MMX: "-target-feature" "+mmx" "-target-feature" "+3dnow" "-target-feature" "+3dnowa"
 // NO-MMX: "-target-feature" "-mmx" "-target-feature" "-3dnow" "-target-feature" "-3dnowa"
 

llvm/lib/Target/X86/X86CallingConv.td

 def RetCC_X86_32_C : CallingConv<[
   // The X86-32 calling convention returns FP values in FP0, unless marked
   // with "inreg" (used here to distinguish one kind of reg from another,
   // weirdly; this is really the sse-regparm calling convention) in which
   // case they use XMM0, otherwise it is the same as the common X86 calling
   // conv.
   CCIfInReg<CCIfSubtarget<"hasSSE2()",
     CCIfType<[f32, f64], CCAssignToReg<[XMM0,XMM1,XMM2]>>>>,
+  CCIfNotSubtarget<"hasX87()",
+    CCIfType<[f32], CCAssignToReg<[AX, DX, CX]>>>,
+  CCIfNotSubtarget<"hasX87()",
+    CCIfType<[f64], CCAssignToReg<[EAX, EDX, ECX]>>>,
   CCIfType<[f32,f64], CCAssignToReg<[FP0, FP1]>>,
   CCIfType<[f16], CCAssignToReg<[XMM0,XMM1,XMM2]>>,
   CCDelegateTo<RetCC_X86Common>
 ]>;
 
 // X86-32 FastCC return-value convention.
 def RetCC_X86_32_Fast : CallingConv<[
   // The X86-32 fastcc returns 1, 2, or 3 FP values in XMM0-2 if the target has
   // SSE2.
   // This can happen when a float, 2 x float, or 3 x float vector is split by
   // target lowering, and is returned in 1-3 sse regs.
   CCIfType<[f32], CCIfSubtarget<"hasSSE2()", CCAssignToReg<[XMM0,XMM1,XMM2]>>>,

llvm/test/CodeGen/X86/no-x87.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=i686-- | FileCheck %s -check-prefix=X87
+; RUN: llc < %s -mtriple=i686-- -mattr=-x87 | FileCheck %s -check-prefix=NOX87
+
+define float @foo(float %a, float %b) nounwind {
+; X87-LABEL: foo:
+; X87:       # %bb.0: # %entry
+; X87-NEXT:    flds {{[0-9]+}}(%esp)
+; X87-NEXT:    retl
+;
+; NOX87-LABEL: foo:
+; NOX87:       # %bb.0: # %entry
+; NOX87-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; NOX87-NEXT:    retl
+entry:
+  ret float %b
+}