Workaround WSL misconfiguring the x87 FPU

jart · Nov 2, 2022 · d6ff4c7 · d6ff4c7
1 parent f44d887
commit d6ff4c7
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Showing 2 changed files with 26 additions and 19 deletions.
diff --git a/libc/runtime/cosmo.S b/libc/runtime/cosmo.S
@@ -44,6 +44,32 @@ cosmo:	push	%rbp
 	mov	%eax,%r12d
 #endif /* SYSDEBUG */
 
+//	Windows always initializes FPU to douuble precision.
+//	WSL breaks Linux ABI by initializing FPU to double precision.
+//	This code makes long double long again.
+//
+//	@see System V Application Binary Interface NexGen32e Architecture
+//	     Processor Supplement, Version 1.0, December 5th, 2018
+//	     Section 3.4.1: Initial Stack and Register State
+	fldcw	1f(%rip)
+	.rodata
+	.align	2
+//	8087 FPU Control Word
+//	 IM: Invalid Operation ───────────────┐
+//	 DM: Denormal Operand ───────────────┐│
+//	 ZM: Zero Divide ───────────────────┐││
+//	 OM: Overflow ─────────────────────┐│││
+//	 UM: Underflow ───────────────────┐││││
+//	 PM: Precision ──────────────────┐│││││
+//	 PC: Precision Control ───────┐  ││││││
+//	  {float,∅,double,long double}│  ││││││
+//	 RC: Rounding Control ──────┐ │  ││││││
+//	  {even, →-∞, →+∞, →0}      │┌┤  ││││││
+//	                           ┌┤││  ││││││
+//	                          d││││rr││││││
+1:	.short	0b00000000000000000001101111111
+	.previous
+
 #ifdef __FAST_MATH__
 	push	%rax
 	stmxcsr	(%rsp)

diff --git a/libc/runtime/winmain.greg.c b/libc/runtime/winmain.greg.c
@@ -76,24 +76,6 @@ static const short kConsoleModes[3] = {
         kNtEnableVirtualTerminalProcessing,
 };
 
-forceinline void MakeLongDoubleLongAgain(void) {
-  /* 8087 FPU Control Word
-      IM: Invalid Operation ───────────────┐
-      DM: Denormal Operand ───────────────┐│
-      ZM: Zero Divide ───────────────────┐││
-      OM: Overflow ─────────────────────┐│││
-      UM: Underflow ───────────────────┐││││
-      PM: Precision ──────────────────┐│││││
-     PC: Precision Control ────────┐  ││││││
-      {float,∅,double,long double} │  ││││││
-     RC: Rounding Control ───────┐ │  ││││││
-      {even, →-∞, →+∞, →0}       │┌┤  ││││││
-                                ┌┤││  ││││││
-                               d││││rr││││││*/
-  int x87cw = 0b0000000000000000001101111111;
-  asm volatile("fldcw\t%0" : /* no outputs */ : "m"(x87cw));
-}
-
 // https://nullprogram.com/blog/2022/02/18/
 static inline char16_t *MyCommandLine(void) {
   void *cmd;
@@ -259,7 +241,6 @@ __msabi textwindows int64_t WinMain(int64_t hInstance, int64_t hPrevInstance,
   if (__strstr16(cmdline, u"--strace")) ++__strace;
 #endif
   NTTRACE("WinMain()");
-  MakeLongDoubleLongAgain();
   if (_weaken(WinSockInit)) _weaken(WinSockInit)();
   if (_weaken(WinMainForked)) _weaken(WinMainForked)();
   WinMainNew(cmdline);