[PATCH] D26151: RegCall - Handling long double arguments

[Oren Ben Simhon via llvm-commits]

oren_ben_simhon added inline comments.


================
Comment at: lib/Target/X86/X86FloatingPoint.cpp:973
   assert(STReturns == 0 || (isMask_32(STReturns) && N <= 2));
+  StackTop = 0;
 
----------------
ahatanak wrote:
> oren_ben_simhon wrote:
> > ahatanak wrote:
> > > oren_ben_simhon wrote:
> > > > ahatanak wrote:
> > > > > I'm not sure what this comment means.
> > > > > 
> > > > > It seems like you are clearing StackTop because the first FP register can be used to pass an argument when the calling convention is regcall. Is that correct?
> > > > The problem occurs when FP0 was passed as argument and we try to return a parameter in FP0 as well.
> > > > In that case the StackTop points to 1 and we get to the following loop and we call pushreg(0) when FP0 is already assigned and StackTop is 1. 
> > > > 
> > > > So what actually happened before pushreg:    
> > > >            Stack[] = { 0, ...}
> > > >            RegMap[] = { 0, ...}
> > > > While after pushreg[0] (No stacktop reset):      
> > > >            Stack[] = { 0, 1..}
> > > >            RegMap[] = { 1...}
> > > > 
> > > > The issue causes debug assertion in line 169 to fail because then regmap[stack[0]] != 0.
> > > > 
> > > > I changed the way I handle the issue.
> > > > Instead of reseting the StackTop to zero, I do not push registers if they were already pushed into the stack.
> > > Wouldn't this push an extra register when a function taking a long double argument and returning complex long double were called (in which case N==2 because it's returned in st0 and st1)?
> > AFAIK, Long doubles are saved inside a single ST register (size of 80 bit). 
> > So in the case of one FP argument and one FP retruned value, I expect N to be 1. 
> > Meaning, The value will be returned in ST0.
> When the following code is compiled with clang, the complex long double return value is returned in two registers. N is 2 and StackTop is 1 when the call to foo3 is visited in FPS::handleCall:
> 
> ```
> typedef long double _Complex CLD;
> long double d0;
> 
> CLD __regcall foo3(long double a);
> 
> CLD __regcall  foo2(void) {
>   return foo3(d0);
> }
> ```
> 
> $ clang -cc1  -ffreestanding -triple=x86_64-pc-linux-gnu -o - -S  test1.c
> 
> Do you know why this is happening?
You are correct. It acts according the default x64 calling convention, which returns COMPLEX_X87 in two registers.
The updated condition below will not break this behaviour and still two registers will be assigned.


Repository:
  rL LLVM

https://reviews.llvm.org/D26151