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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:black">>
</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">> On Thu Dec 18 2014 at 1:21:53 PM Daniel Sanders <<a href="mailto:Daniel.Sanders@imgtec.com">Daniel.Sanders@imgtec.com</a>> wrote:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:black">> > > 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.<br>
> > 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.<br>
> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">> The point I was making is that the rationale for this patch is completely mistaken. i.e. gcc does use $at for code<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">> generation so unless gcc is adding an implicit clobber of $at in inline asm then it's not going to be correct.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">GCC goes further than an implicit clobber. It has promised not to keep any values in $at.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Eric Christopher [mailto:echristo@gmail.com]
<br>
<b>Sent:</b> 18 December 2014 21:29<br>
<b>To:</b> Daniel Sanders; reviews+D6638+public+9dbc664d9cbf7b31@reviews.llvm.org; Toma Tabacu<br>
<b>Cc:</b> cfe-commits@cs.uiuc.edu<br>
<b>Subject:</b> Re: [PATCH] [mips] Always clobber $1 for MIPS inline asm.<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu Dec 18 2014 at 1:21:53 PM Daniel Sanders <<a href="mailto:Daniel.Sanders@imgtec.com">Daniel.Sanders@imgtec.com</a>> wrote:<o:p></o:p></p>
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<div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black">> 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.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black">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.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black"><br>
> This is a pretty weird hack, I think you could instead just look for uses of $at or macro instructions<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black">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.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">So, why would you put this patch in again?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-eric<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black"><br>
> (unless that other patch recently just set nomacro for inline assembly).<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black">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.<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-family:"Tahoma","sans-serif";color:black">From:</span></b><span style="font-family:"Tahoma","sans-serif";color:black"> Eric Christopher [<a href="mailto:echristo@gmail.com" target="_blank">echristo@gmail.com</a>]<br>
<b>Sent:</b> 18 December 2014 20:29<br>
<b>To:</b> <a href="mailto:reviews%2BD6638%2Bpublic%2B9dbc664d9cbf7b31@reviews.llvm.org" target="_blank">
reviews+D6638+public+9dbc664d9cbf7b31@reviews.llvm.org</a>; Toma Tabacu; Daniel Sanders<br>
<b>Cc:</b> <a href="mailto:cfe-commits@cs.uiuc.edu" target="_blank">cfe-commits@cs.uiuc.edu</a><br>
<b>Subject:</b> Re: [PATCH] [mips] Always clobber $1 for MIPS inline asm.</span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="color:black">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.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="color:black">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).<o:p></o:p></span></p>
</div>
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<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="color:black">-eric<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<div>
<p class="MsoNormal"><span style="color:black">On Thu Dec 18 2014 at 8:06:36 AM Toma Tabacu <<a href="mailto:toma.tabacu@imgtec.com" target="_blank">toma.tabacu@imgtec.com</a>> wrote:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">Removed FIXME, as suggested in the review.<br>
<br>
<br>
<a href="http://reviews.llvm.org/D6638" target="_blank">http://reviews.llvm.org/D6638</a><br>
<br>
Files:<br>
lib/Basic/Targets.cpp<br>
test/CodeGen/mips-constraint-regs.c<br>
test/CodeGen/mips-constraints-mem.c<br>
test/CodeGen/mips-inline-asm-modifiers.c<br>
test/CodeGen/mult-alt-generic.c<br>
<br>
Index: lib/Basic/Targets.cpp<br>
===================================================================<br>
--- lib/Basic/Targets.cpp<br>
+++ lib/Basic/Targets.cpp<br>
@@ -5749,8 +5749,7 @@<br>
}<br>
<br>
const char *getClobbers() const override {<br>
- // FIXME: Implement!<br>
- return "";<br>
+ return "~{$1}";<br>
}<br>
<br>
bool handleTargetFeatures(std::vector<std::string> &Features,<br>
Index: test/CodeGen/mips-constraint-regs.c<br>
===================================================================<br>
--- test/CodeGen/mips-constraint-regs.c<br>
+++ test/CodeGen/mips-constraint-regs.c<br>
@@ -9,7 +9,7 @@<br>
// 'c': 16 bit address register for Mips16, GPR for all others<br>
// I am using 'c' to constrain both the target and one of the source<br>
// registers. We are looking for syntactical correctness.<br>
- // 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]+}}<br>
+ // 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]+}}<br>
int __s, __v = 17;<br>
int __t;<br>
__asm__ __volatile__(<br>
@@ -20,7 +20,7 @@<br>
// 'l': lo register<br>
// We are making it clear that destination register is lo with the<br>
// use of the 'l' constraint ("=l").<br>
- // CHECK: %{{[0-9]+}} = call i32 asm sideeffect "mtlo $1 \0A\09\09", "=l,r,~{lo}"(i32 %{{[0-9]+}}) [[NUW]], !srcloc !{{[0-9]+}}<br>
+ // CHECK: %{{[0-9]+}} = call i32 asm sideeffect "mtlo $1 \0A\09\09", "=l,r,~{lo},~{$1}"(i32 %{{[0-9]+}}) [[NUW]], !srcloc !{{[0-9]+}}<br>
int i_temp = 44;<br>
int i_result;<br>
__asm__ __volatile__(<br>
@@ -32,7 +32,7 @@<br>
// 'x': Combined lo/hi registers<br>
// We are specifying that destination registers are the hi/lo pair with the<br>
// use of the 'x' constraint ("=x").<br>
- // 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]+}}<br>
+ // 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]+}}<br>
int i_hi = 3;<br>
int i_lo = 2;<br>
long long ll_result = 0;<br>
Index: test/CodeGen/mips-constraints-mem.c<br>
===================================================================<br>
--- test/CodeGen/mips-constraints-mem.c<br>
+++ test/CodeGen/mips-constraints-mem.c<br>
@@ -9,7 +9,7 @@<br>
// 'R': An address that can be used in a non-macro load or stor'<br>
// This test will result in the higher and lower nibbles being<br>
// switched due to the lwl/lwr instruction pairs.<br>
- // 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,<br>
+ // 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,<br>
<br>
int c = 0xffbbccdd;<br>
<br>
Index: test/CodeGen/mips-inline-asm-modifiers.c<br>
===================================================================<br>
--- test/CodeGen/mips-inline-asm-modifiers.c<br>
+++ test/CodeGen/mips-inline-asm-modifiers.c<br>
@@ -7,9 +7,9 @@<br>
<br>
typedef int v4i32 __attribute__((vector_size(16)));<br>
<br>
- // 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,<br>
- // 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,<br>
- // CHECK: %{{[0-9]+}} = call <4 x i32> asm "ldi.w ${0:w},1", "=f"<br>
+ // 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,<br>
+ // 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,<br>
+ // CHECK: %{{[0-9]+}} = call <4 x i32> asm "ldi.w ${0:w},1", "=f,~{$1}"<br>
int b[8] = {0,1,2,3,4,5,6,7};<br>
int main()<br>
{<br>
Index: test/CodeGen/mult-alt-generic.c<br>
===================================================================<br>
--- test/CodeGen/mult-alt-generic.c<br>
+++ test/CodeGen/mult-alt-generic.c<br>
@@ -17,7 +17,7 @@<br>
// CHECK: @single_m<br>
void single_m()<br>
{<br>
- // CHECK: call void asm "foo $1,$0", "=*m,*m[[CLOBBERS:[a-zA-Z0-9@%{},~_ ]*\"]](i32* {{[a-zA-Z0-9@%]+}}, i32* {{[a-zA-Z0-9@%]+}})<br>
+ // CHECK: call void asm "foo $1,$0", "=*m,*m[[CLOBBERS:[a-zA-Z0-9@%{},~_$ ]*\"]](i32* {{[a-zA-Z0-9@%]+}}, i32* {{[a-zA-Z0-9@%]+}})<br>
asm("foo %1,%0" : "=m" (mout0) : "m" (min1));<br>
}<br>
<br>
EMAIL PREFERENCES<br>
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http://reviews.llvm.org/settings/panel/emailpreferences/</a><br>
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