[PATCH] [mips] Always clobber $1 for MIPS inline asm.
Daniel Sanders
Daniel.Sanders at imgtec.com
Thu Dec 18 13:52:41 PST 2014
> > On Thu Dec 18 2014 at 1:21:53 PM Daniel Sanders <Daniel.Sanders at imgtec.com<mailto:Daniel.Sanders at imgtec.com>> wrote:
> > > For the record, gcc does use $at for code generation, take a look at mips_print_operand for the @ symbol and then look at all of the cases it's being used.
> > That's correct. However, LLVM currently doesn't use $at for codegen and quite a bit of effort has been put in to making that the case. I'd like to retain this advantage over GCC if I can.
> That's not what the original mail said and there's no real reason to avoid using $at. I'm not sure I understand this rationale.
I see where we're confusing you, Toma and I aren't being precise enough. In GCC, $at is a reserved register. As you say, it does use $at in generated code but as far as I know, it only does so as a very short lived temporary. In LLVM, $at is an allocatable register and is freely used in all code generation.
In GCC inline assembly, $at is still a reserved register and users have taken advantage of this. In LLVM, $at needs to be clobbered by inline assembly so that the users usage of $at doesn't break the compiler generated code.
> The point I was making is that the rationale for this patch is completely mistaken. i.e. gcc does use $at for code
> generation so unless gcc is adding an implicit clobber of $at in inline asm then it's not going to be correct.
GCC goes further than an implicit clobber. It has promised not to keep any values in $at.
From: Eric Christopher [mailto:echristo at gmail.com]
Sent: 18 December 2014 21:29
To: Daniel Sanders; reviews+D6638+public+9dbc664d9cbf7b31 at reviews.llvm.org; Toma Tabacu
Cc: cfe-commits at cs.uiuc.edu
Subject: Re: [PATCH] [mips] Always clobber $1 for MIPS inline asm.
On Thu Dec 18 2014 at 1:21:53 PM Daniel Sanders <Daniel.Sanders at imgtec.com<mailto:Daniel.Sanders at imgtec.com>> wrote:
> For the record, gcc does use $at for code generation, take a look at mips_print_operand for the @ symbol and then look at all of the cases it's being used.
That's correct. However, LLVM currently doesn't use $at for codegen and quite a bit of effort has been put in to making that the case. I'd like to retain this advantage over GCC if I can.
That's not what the original mail said and there's no real reason to avoid using $at. I'm not sure I understand this rationale.
> This is a pretty weird hack, I think you could instead just look for uses of $at or macro instructions
I think inspecting the assembly text to automatically discover clobbers would be a nice feature but it would mean that GCC wouldn't correctly handle inline assembly that works fine on LLVM. I don't think we need to do that now. The goal at the moment is to become compatible with GCC's inline assembly so that LLVM-compiled Linux works for Mips.
The point I was making is that the rationale for this patch is completely mistaken. i.e. gcc does use $at for code generation so unless gcc is adding an implicit clobber of $at in inline asm then it's not going to be correct.
So, why would you put this patch in again?
-eric
> (unless that other patch recently just set nomacro for inline assembly).
The other patch effectively prepends '.set macro', '.set reorder', and '.set at' to the inline assembly to match the values that GCC is using for inline assembly (and codegen in general). After the inline assembly it reverts back to the mode LLVM normally uses. This is because there are quite a few inline assembly blocks in the linux kernel that expect .set macro/reorder/at.
________________________________
From: Eric Christopher [echristo at gmail.com<mailto:echristo at gmail.com>]
Sent: 18 December 2014 20:29
To: reviews+D6638+public+9dbc664d9cbf7b31 at reviews.llvm.org<mailto:reviews%2BD6638%2Bpublic%2B9dbc664d9cbf7b31 at reviews.llvm.org>; Toma Tabacu; Daniel Sanders
Cc: cfe-commits at cs.uiuc.edu<mailto:cfe-commits at cs.uiuc.edu>
Subject: Re: [PATCH] [mips] Always clobber $1 for MIPS inline asm.
For the record, gcc does use $at for code generation, take a look at mips_print_operand for the @ symbol and then look at all of the cases it's being used.
This is a pretty weird hack, I think you could instead just look for uses of $at or macro instructions (unless that other patch recently just set nomacro for inline assembly).
-eric
On Thu Dec 18 2014 at 8:06:36 AM Toma Tabacu <toma.tabacu at imgtec.com<mailto:toma.tabacu at imgtec.com>> wrote:
Removed FIXME, as suggested in the review.
http://reviews.llvm.org/D6638
Files:
lib/Basic/Targets.cpp
test/CodeGen/mips-constraint-regs.c
test/CodeGen/mips-constraints-mem.c
test/CodeGen/mips-inline-asm-modifiers.c
test/CodeGen/mult-alt-generic.c
Index: lib/Basic/Targets.cpp
===================================================================
--- lib/Basic/Targets.cpp
+++ lib/Basic/Targets.cpp
@@ -5749,8 +5749,7 @@
}
const char *getClobbers() const override {
- // FIXME: Implement!
- return "";
+ return "~{$1}";
}
bool handleTargetFeatures(std::vector<std::string> &Features,
Index: test/CodeGen/mips-constraint-regs.c
===================================================================
--- test/CodeGen/mips-constraint-regs.c
+++ test/CodeGen/mips-constraint-regs.c
@@ -9,7 +9,7 @@
// 'c': 16 bit address register for Mips16, GPR for all others
// I am using 'c' to constrain both the target and one of the source
// registers. We are looking for syntactical correctness.
- // CHECK: %{{[0-9]+}} = call i32 asm sideeffect "addi $0,$1,$2 \0A\09\09", "=c,c,I"(i32 %{{[0-9]+}}, i32 %{{[0-9]+}}) [[NUW:#[0-9]+]], !srcloc !{{[0-9]+}}
+ // CHECK: %{{[0-9]+}} = call i32 asm sideeffect "addi $0,$1,$2 \0A\09\09", "=c,c,I,~{$1}"(i32 %{{[0-9]+}}, i32 %{{[0-9]+}}) [[NUW:#[0-9]+]], !srcloc !{{[0-9]+}}
int __s, __v = 17;
int __t;
__asm__ __volatile__(
@@ -20,7 +20,7 @@
// 'l': lo register
// We are making it clear that destination register is lo with the
// use of the 'l' constraint ("=l").
- // CHECK: %{{[0-9]+}} = call i32 asm sideeffect "mtlo $1 \0A\09\09", "=l,r,~{lo}"(i32 %{{[0-9]+}}) [[NUW]], !srcloc !{{[0-9]+}}
+ // CHECK: %{{[0-9]+}} = call i32 asm sideeffect "mtlo $1 \0A\09\09", "=l,r,~{lo},~{$1}"(i32 %{{[0-9]+}}) [[NUW]], !srcloc !{{[0-9]+}}
int i_temp = 44;
int i_result;
__asm__ __volatile__(
@@ -32,7 +32,7 @@
// 'x': Combined lo/hi registers
// We are specifying that destination registers are the hi/lo pair with the
// use of the 'x' constraint ("=x").
- // CHECK: %{{[0-9]+}} = call i64 asm sideeffect "mthi $1 \0A\09\09mtlo $2 \0A\09\09", "=x,r,r"(i32 %{{[0-9]+}}, i32 %{{[0-9]+}}) [[NUW]], !srcloc !{{[0-9]+}}
+ // CHECK: %{{[0-9]+}} = call i64 asm sideeffect "mthi $1 \0A\09\09mtlo $2 \0A\09\09", "=x,r,r,~{$1}"(i32 %{{[0-9]+}}, i32 %{{[0-9]+}}) [[NUW]], !srcloc !{{[0-9]+}}
int i_hi = 3;
int i_lo = 2;
long long ll_result = 0;
Index: test/CodeGen/mips-constraints-mem.c
===================================================================
--- test/CodeGen/mips-constraints-mem.c
+++ test/CodeGen/mips-constraints-mem.c
@@ -9,7 +9,7 @@
// 'R': An address that can be used in a non-macro load or stor'
// This test will result in the higher and lower nibbles being
// switched due to the lwl/lwr instruction pairs.
- // CHECK: %{{[0-9]+}} = call i32 asm sideeffect "lwl $0, 1 + $1\0A\09lwr $0, 2 + $1\0A\09", "=r,*R"(i32* %{{[0-9,a-f]+}}) #1,
+ // CHECK: %{{[0-9]+}} = call i32 asm sideeffect "lwl $0, 1 + $1\0A\09lwr $0, 2 + $1\0A\09", "=r,*R,~{$1}"(i32* %{{[0-9,a-f]+}}) #1,
int c = 0xffbbccdd;
Index: test/CodeGen/mips-inline-asm-modifiers.c
===================================================================
--- test/CodeGen/mips-inline-asm-modifiers.c
+++ test/CodeGen/mips-inline-asm-modifiers.c
@@ -7,9 +7,9 @@
typedef int v4i32 __attribute__((vector_size(16)));
- // CHECK: %{{[0-9]+}} = call i32 asm ".set noreorder;\0Alw $0,$1;\0A.set reorder;\0A", "=r,*m"(i32* getelementptr inbounds ([8 x i32]* @b, i32 {{[0-9]+}}, i32 {{[0-9]+}})) #2,
- // CHECK: %{{[0-9]+}} = call i32 asm "lw $0,${1:D};\0A", "=r,*m"(i32* getelementptr inbounds ([8 x i32]* @b, i32 {{[0-9]+}}, i32 {{[0-9]+}})) #2,
- // CHECK: %{{[0-9]+}} = call <4 x i32> asm "ldi.w ${0:w},1", "=f"
+ // CHECK: %{{[0-9]+}} = call i32 asm ".set noreorder;\0Alw $0,$1;\0A.set reorder;\0A", "=r,*m,~{$1}"(i32* getelementptr inbounds ([8 x i32]* @b, i32 {{[0-9]+}}, i32 {{[0-9]+}})) #2,
+ // CHECK: %{{[0-9]+}} = call i32 asm "lw $0,${1:D};\0A", "=r,*m,~{$1}"(i32* getelementptr inbounds ([8 x i32]* @b, i32 {{[0-9]+}}, i32 {{[0-9]+}})) #2,
+ // CHECK: %{{[0-9]+}} = call <4 x i32> asm "ldi.w ${0:w},1", "=f,~{$1}"
int b[8] = {0,1,2,3,4,5,6,7};
int main()
{
Index: test/CodeGen/mult-alt-generic.c
===================================================================
--- test/CodeGen/mult-alt-generic.c
+++ test/CodeGen/mult-alt-generic.c
@@ -17,7 +17,7 @@
// CHECK: @single_m
void single_m()
{
- // CHECK: call void asm "foo $1,$0", "=*m,*m[[CLOBBERS:[a-zA-Z0-9@%{},~_ ]*\"]](i32* {{[a-zA-Z0-9@%]+}}, i32* {{[a-zA-Z0-9@%]+}})
+ // CHECK: call void asm "foo $1,$0", "=*m,*m[[CLOBBERS:[a-zA-Z0-9@%{},~_$ ]*\"]](i32* {{[a-zA-Z0-9@%]+}}, i32* {{[a-zA-Z0-9@%]+}})
asm("foo %1,%0" : "=m" (mout0) : "m" (min1));
}
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